Do you have a preference for Kingdom versus Empire? Strategic casters or tactical casters? What type of mage do you want to play? How much versatility do you want said mage to have? A custom Mage sovereign should probably have Brilliant and Tactician; at the very least, the experience bonus from Brilliant is worthwhile, and you can never go wrong with more initiative.
You should perhaps look at the spells available in each path and decide what you want to play with, rather than having me tell you. Regardless, if you're looking for a damage caster, I would say that you more or less cannot go wrong with Fire 2/3 (for Flame Dart at Fire 2 and Fireball at Fire 3), Water 4 for Blizzard (possibly the best AOE damage spell in the game), or Death for Drain Life at Death 3, Touch of Entropy at Death 4, and Kill at Death 5. Fire is your minimum investment damage path, and also note that in the early stages of the game, Burning Hands is both cheap enough to cast and reasonably effective when compared to normal weapons, especially if you can manage to hit two or three enemies with it (Burning Hands is also the only first level damage spell in the game). Fire should be paired with either Water or Death so that you have an alternative damage spell to go to if and when you come across units with high fire resistance; Water is in my opinion more attractive for a pure damage caster just because of how good Blizzard is against multi-figure units, though Death is much better against single powerful targets like dragons or the bosses of Wildlands (especially once you get to Kill). Remember that Blizzard is relatively ineffective against single-figure high-health units and targets with lots of cold resistance. Air and Earth are possible schools for damage casters, although neither is exceptional; Air has only one useful damage spell, Storm, at Air 3, as Thunderstrike (also at Air 3) requires you to teleport your caster next to an enemy in order to make use of the damage, though the teleport effect is useful enough on its own and Storm unfortunately cannot be aimed, while Earth has two relatively ineffective damage spells in Shockwave at Earth 3 (10 +2 per Earth Shard physical damage to units adjacent to the caster) and Shatter at Earth 4 (target loses 25% of its health and armor; this is more useful when used as a curse than as pure damage). Life has no damage spells.
If you're looking for a tactical support caster, about the only path that you can go wrong in taking is Fire, and even then that's useful for something to do when you don't have a reason to cast a blessing or a curse (fire does offer Focus, which improves the caster's damage - invaluable for your damage mage - and Burning Blade for any champions/henchmen in the fight, which are both useful but not anything to write home about, as far as support spells go). Water and Air each have an excellent support spell available from the very start of the game (Air 1 for Haste and Water 1 for Slow, both of which have very low casting costs and affect initiative). Air 2 opens up Guardian Wind, which is a fairly good counter to ranged attacks, and Air 4 offers Titan's Breath, which will cause any enemy unit that fails the spell resistance check to lose its next turn (and possibly suffer guaranteed hits, should you be able to get a unit into striking distance). Death offers a lot of good to excellent curses; Wither at Death 1 reduces enemy attack scores by 2 plus however many Death Shards you have (if you go this route, consider taking Death Worship as a faction trait or play Resoln so that you can convert the shards you find over to Death, as this will empower both Wither, to remove the threat posed by the enemy armies, and Dirge of Ceresa, to make the threat removal permanent, in addition to granting Grave Seal for guaranteed critical hits on the affected target and Infection for spreading single-target curses to the entire army), Curse/Mass Curse at Death 1/3 to set your target's defense score to zero for three turns and lay on the damage, Blind at Death 2 for reducing enemy damage potential by reducing accuracy (works well for dodge-based units, not so well on defense-based units, and in my opinion isn't as good as Wither, though it's a useful complement to that spell and arguably better against units with very high attack scores), Shadowbolt at Death 3 for a now-negligible amount of damage and a guaranteed reduction in spell resistance (useful for getting attacks to stick to a high-resistance target but not for much else), and Siphon Strength at Death 3 which is more of a spell that a Warrior/Assassin/Defender with spellcasting abilities would use (still not useless for a support caster, but Wither and Blind have a similar effect and are more accessible, and Wither is potentially much better). Earth has Stoneskin at Earth 2 which is more useful for warrior-mages than for pure spellcasters, Diamondskin at Earth 3 which is hellishly expensive but makes the target invlunerable for three turns, Fracture also at Earth 3 which is a weaker and much more expensive Curse with a moderate amount of damage tacked on, and Giant Form at Earth 5 which gives a target unit an excellent bonus to its attack score (Growth at Life 3 is a weaker, more accessible, cheaper version with an added drawback). Life 2 gives you Heal and Life 4 gives you Wellspring (mass heal), both of which are excellent for keeping an army fighting (Wellspring additionally has a strategic version and can be used to heal up a battered army quickly after a tough fight), as well as Growth and Shrink at Life 3. Growth is a useful unit blessing that improves the attack score at the cost of the dodge score, while Shrink is a useful curse reducing the enemy's attack score but improving their dodge score. Shrink is probably less effective overall than Wither (especially if you have a big stack of Death Shards).
Summoners don't need any particular magic paths, as all the summon spells come from Path of the Mage traits.
Long story short: if you want tactical damage casters, you want Fire 2 or Fire 3, Water 4, or Death 3/4/5 as your main (or main + secondary) magic path. If you want tactical support casters, you want Air 1/2/4, Water 1, Life 2/3/4, or Death 1/2/3 as your main (or main + secondary) magic path. If you're looking for strategic support, Water + Earth can more or less lock down an army indefinitely (note that these do not need to be on the same caster, and that 2 Water casters or 2 Earth casters can accomplish the same thing), Fire and Water both have good strategic damage spells, Earth offers a city attack, Life offers strategic-level healing, Air/Fire/Earth/Life offer city enchantments that offer great benefits to trained units (particularly Air and Fire), Air offers a city enchantment boosting your gold income, and Death offers the only research per essence spell left in the game (Pit of Madness at Death 4).
Earth is the only magic path which I would not recommend taking on a primary spellcaster; most of what it does is accomplished more cheaply or more easily by other magic paths, and only Diamond Skin (which is incredibly expensive) and Giant Form are superior to the alternatives in the other paths, and the effect of Diamond Skin can technically be replicated by Wither if you have enough Death Shards except that it applies to your entire army instead of just one unit. Air is a good go-to path if you're looking for a mage who can do a little of everything but won't be investing a lot of levels into the elemental schools. Fire is your minimum-investment damage school and has reasonable options for early-game damage spellcasting as well as mid to late-game single-target and AOE damage spellcasting, while Blizzard (Water 4 or scrolls from loot or scrolls purchased for 100 gold each from a Scroll Scribe Conclave) or Horrific Wail (Greater Necromancy Mage trait, secondary summon line for Empires) or Dirge of Ceresa (Death Worship faction trait, though Dirge of Ceresa is sovereign-only and only usually becomes an army killer if you convert a lot of shards over to Death Shards) are your single-spell army killers. I would suggest picking one role (damage or or cursing/blessing or summoning/blessing) and focusing on that until you develop all the traits necessary for that primary focus, which should tell you which elemental paths you want to have and develop. I would further suggest that you take no more than two elemental paths, nor would I suggest starting more than one elemental path at level 2 (if you do start a path at level 2, I would suggest that you either pick a path that gives you all the spells you want out of it by level 2, or a path that you eventually want to have at rank 4 or 5). Death + Air and Death + Water are excellent support caster choices, as both Air and Water only really need the first rank to be effective and the Kill spell at Death 5 represents an excellent use for all that spell mastery a curse caster has been stacking, as it's a single-target damage spell that is unaffected by Evoker traits and is heavily reliant on spell mastery, and Death has plenty of other reasons for you to improve your caster's skill within it (Death 2/3 is perhaps the weakest point for Death, as Mass Curse is not necessarily that much better than Curse and Death 2 doesn't offer anything great, and Drain Life is a bit weak in comparison to Flame Dart or Fireball, which you could have for a similar investment of traits; Pit of Madness at Death 4 is worth taking Death 2 and Death 3 for regardless of whether or not you're playing with the other toys you get from it, and Kill at Death 5 is only matched by Mana Blast or a high-level caster's Flame Dart for single-target damage).
As far as races go, Amarians are the only race which really has an innate bonus to spellcasting - they gain 1 initiative per Fire Shard and 1 spell mastery per Water Shard - while Ironeers are the only race which you really ought to avoid for tactical spellcasters (their tactical spells cost 25% more mana, though this has no impact on their strategic-level spellcasting). Wraiths have a unique trait available in unit design which allows your units to produce mana for your casters by getting the finishing blow on an enemy, though this can take careful management of the battle. Men offer Henchmen, which grant you limited access to trainable mages (importantly, a Race of Men Empire has access to Life Magic for healing) as well as a racial experience bonus, and given that mages tend to need to reach at least the low teens before they're really all that useful this is a nice, though not essential, bonus (blessing casters and, to a lesser extent, summoners are unique in that they're effective even at low levels, as they do not need to make spell resistance checks to apply full damage like a damage caster does or apply a curse like a curse caster does; blessings will always stick to your own units, and Haste, one of the best blessings in the game, is available to any caster with at least Air 1 and 4 mana to spend on it; Life blessing casters need a little more investment as you need Life 2 and 16 mana for Heal, though this is also available from the start of the game if you design your sovereign for it).
Death Worship and Flesh-bound Tome both offer useful spells (Death Worship offers Grave Seal and Infection, which are excellent for curse spellcasters, Dirge of Ceresa, which is a mediocre to exceptional mass damage spell depending on how many Death Shards you have, and Corruption, which allows you to turn any shard in your zone of control into a Death Shard; Flesh-bound Tome offers Cull the Weak, which heals the caster for 20 life and generates 20 mana at the cost of killing one of your own units, Candlecloak, which is not that great, and Consume, which gives you 200 mana now at the cost of losing whatever you'd have gained from the shard you destroyed; Cull the Weak stacks well with the Undead and Quendar as both can mass lots of cheap garbage units - slaves for Quendar and skeletons for Undead - that you likely won't mind trading in for the 20 free mana, though Undead suffer from an initiative penalty which is bad news for any caster and Quendar have no real bonuses for spellcasting); the Dead also have a faction trait that gives you some fun spells, including one that summons an army of skeletons, though I cannot recall the trait name at the moment. The faction trait Decalon is useful for allowing you to broaden your spellcasting abilities in the mid- to late-game (if you're designing around this consideration, I would avoid taking Water or Air on the Sovereign as both of these schools are useful in battle at level 1, which is what you'd unlock with a spellbook; Fire and Earth are rather less useful at level 1 by the time you're likely to have access to the spellbooks, and there are no spellbooks for either Life or Death), while Enchanters gets you the Scrying Pool if my memory is correct, and this gets you an extra city enchantment and increases the effects of existing scaling enchantments, useful for adding Meditation to get the mana supply going.
Also note that Savant, which reduces casting time by 1 turn, requires the Prodigy traits which improve spell mastery. If you're looking at building a caster who wants to cast spells that have a casting time, it's worth considering focusing on schools which benefit from going up the Prodigy line, which means looking at curses and damage spells rather than blessings and summons. There's also a mage trait that reduces spell costs in additon to the 25% reduction you get for being a mage and any reduction you get from any equipment you pick up (there's a staff or two with a spell cost discount, and there might be another item or two), which I believe is also in the Prodigy line of traits, so if you're looking at expensive spells like Diamond Skin you may want to consider combining that with a curse school. Damage and curse spellcasters want the prodigy and evoker lines of the Mage tree, summoners want the summoning line, blessing casters don't care what traits you pick up but may have some interest in the spells available from the various -mancy traits.
You should also be aware that there are some things you can do with your empire to further improve your spellcasters, beyond just the faction traits. An Empire faction with at Death 5 caster can cast sacrifice, which turns people into mana at a 1-for-1 rate of exchange and kills half the population of a targeted city; stacking growth bonuses such as you can obtain from Consulates (Outpost upgrade) can let you turn a chosen city into a mana factory (and mass grave) using this spell, and it can let you turn a high-population city or two into mana in a pinch and gives you a use for that level-5 100 unrest city you just took from your enemy (though I think they call this use 'genocide'). Shifting your empire's city balance towards Conclaves can significantly increase your mana income, particularly if you back it up by selecting Oracles (which grant an extra essence for Meditation and some of the Conclave improvements to work on) or Scroll Scribes (which let you turn gold into instant-cast 0-mana Blizzards) at level 3, while the Pyre and the Bathhouse give you 2 Fire shards or 1 Water shard at level 5. Even if you're not going for the Spell of Making victory, the Forge of the Overlord and the towers it requires are good sources of shard power, and Ereog's Tower will get you an Air Shard (though Ereog's Tower is a wonder, so the computer might beat you to it, and it's not as good as the Spell of Making towers), and the magic tree has a repeatable mana production bonus tech (and a Conclave-heavy empire such as you might want to build for spellcasters has a lot of research for stacking the mana production techs and a large base mana income to boost with those techs).
