I'm going to post my balance concerns here, echoing and repeating what others have said (most of this isn't original) but I do believe they interact in a way that has a bigger impact than any one thing.

The Vigilant Minion with his ability to do unavoidable 10 points of damage to all enemies is too powerful for turn 1.  It's honestly too powerful well into mid-game given that your heirs and other magic-wielding champions can all create them.  The army of 5 minions can do way too much damage too quickly and destroy almost all enemies in 1 turn.  And if you thought the mana pool would act as a limiter, at the end game it does not as their pools grow with the +Essence buildings.

Magic-casting champions, on the other hand, seem downright weak.  Late game, with 3 fire shards and champions with 300 essence and over 30 intelligence, you still miss regularly and do 8 or 10 points of damage with Infernal of Magic Missile (err, Dart.)  I thought this was designed to provide a way for melee combat to matter in the endgame, but I'm not so sure that's right.  My level 20 champions should be able to dish out as much damage with a ranged magic attack as they would do if they focused purely on melee.  However, at the moment that's not the case.  And if you contrast your champions abilities with the Vigilant Minion, you'll find them wildly inferior.  There's not a spell you can research that does the same damage as that level 1 minion ability.  I admit that I haven't tried to get Int up to super high values to see if they matter, so maybe I'm missing something if you get 50+ Int.

I've also found myself with fire shards most of the time (with a handful of Large games played) so it could be the lightning spells are better.  If that's the case, then perhaps I'm asking for the fire spells to be enhanced more than just all spells.

I think the tactical combat buff spells are useless.  Does anybody cast them?  Why would you waste action points and mana on a small Str or Dex buff?  Tactical healing spells are useful in some instances but relatively few (since so many kills are one-hit you don't get much time to cast them.)

There is plenty of gear focused on increasing melee ability, but none on increasing magic ability.  It is disappointing to have to purchase Legendary Plate for all my champions regardless of their role.  Perhaps this is the way to help solve the magic system?  If there was decent +magic gear that influenced your spells I could see having certain champions who healed, others who dealt damage, others who buffed, etc.

When leveling up champions, not all stats are created equal.  First, I'm sure few (if any) points are spent on Charisma.  Given how plentiful gold is you don't really need the ability to cheaply recruit (except perhaps at the beginning.) Second, Strength and Dexterity get multiplied by gear in a way that Constitution and Intelligence do not.  As a result, it always seems more valuable to put points into Str or Dex than it does Con.  And again, as far as I can tell, Int has little actual value compared to Str/Dex.  

Essence is overly modified by, of all things, buildings.  The +100% and +50% buildings mean that in the late game you'll have way more mana than you can really spend.  If only there was something worth spending it on during combat.  As it is, that mana goes to teleporting and strategic spells more than tactical combat spells.   Perhaps there is a defect with the +50% building and you shouldn't be able to build them in every city?  But as it is, I've had most champions at +100 Essence (and one with over 300) because of those buildings.

Creatures mana pools being expanded by buildings feels wrong.  The creatures (either summoned or other) feel balanced around their starting mana pools but are unbalanced by the expansion.  The worst is a dragon, whose mana pool starts at something like 12 (which requires you to carefully control when you use him) but can eventually get to the point where it can use that 96 pt direct damage attack with impunity.  The same is true for the minions and other summoned creatures.

I really do enjoy the game but feel it could be that much better with a few tweaks to balance.