Given that the magic system is slated to be revamped somewhat soon (yay!), here is my take on what I'd like to see happen to the magic system in v1.1 and after.
Since Brad has already partially addressed the "Global Mana Pool" issue (sounds excellent, BTW), I'll leave that to the side and talk about the part of the E:WoM magic system that is getting somewhat less attention: variety. I'm worried that if people expect that simply adding more spells will "make things much better," they may be in for a let-down. The current system, independent of the global mana pool issue or lack of spells issue, lacks strategic depth and "flavor." Right now, I feel more like I'm playing "Elemental: War of Identical Units One-shotting Each Other" rather than "Elemental: War of MAGIC."
Here's what I would do to fix it (stolen liberally from MoM) over time:
General:
+ Spell schools should have 30-40 spells each. The spells in each school that a sovereign may research during the game are randomly selected from this list based on the spell rating ("# spell books") that the sovereign has in that school. Each game should have a "I wonder what I'm going to see this time" feel to it.
+ Spell schools should have general theme, so that choosing that school will have actual gameplay style implications.
+ Spell shool knowledge should be rated on a scale of 0-10, where each numerical value on the scale is a "spell book." Choosing more spellbooks in a school should give access to higher level spells. Choosing fewer books means you get just lower level ones. Spell books can be researched on the magic tree, but are somewhat rare and certainly expensive. This change implies that sovereigns have a choice of "going wide or going deep" during the design phase.
+ Researching "spell levels" directly from the magic screen should go away. One must research all available low level spells first before one can learn the higher level ones. No more bee-lining straight to level three to get the giant. This will fix some balance issues.
+ Each pre-made sovereign (like Procipinee) should have their spell book choices trimmed drastically. Some should "go wide" into 2-4 schools and some should "go deep" into 1-2 school(s). The current (almost total) lack of distinction between sovereigns must go away. For instance, if Procipinee is to keep her presence in all magic schools, she should have 1-3 books in each school. This means she will be tactically flexible but will not have access to world-shaking magics unless she really gets lucky in-game and finds lots of books and researches others. This will add enormous flavor to each sovereign.
Schools and Themes:
+ Good/Life: Strong unit protection and buffing. Some anti-magic and anti-magic-item stuff. Weak direct damage. Strong anti-undead and Black/Evil countermagic. Some defensive and scouting "good" unit/creature summons. Some city buffs for knowledge and defense. Diplomacy buffs. Light/Holy magic.
+ Black/Death: Strong unit debuffing, moderate direct damage and excellent damage-over-time. Curses. Good undead and "evil" unit summoning. Some Good/Life counter-magic. Some land/world/resource corruption. Diseases. Some mana generation through unit sacrifice. Diplomacy debuffs. Shadow magic.
+ Red/Fire: Strongest direct damage. Strong unit buffing on the attack side, balanced by simultaneous debuffs on the defense side. Lots of summoning of strong attack creatures that are somewhat weak on defense. Strong land destruction. General chaos theme. Luck/Chance manipulation. Some Blue/Water countermagic.
+ Blue/Water: Weak to moderate direct damage using water/psionics. Strong illusion and phantasm magic. Strong countermagic. Water movement buffs, and anti-magic unit buffs. A few summoning spells involving water creatures. Extremely strong Red/Fire countermagic.
+ Green/Earth: Strong unit protection and buffing. Weak to moderate direct damage. Lots of strong unit summoning. Some food production buffs and land modification/healing spells to counter Fire/Death schools.
+ Gray/Air: Strong movement and speed enhancing spells and buffs, including teleport and flight. Moderate to good direct damage involving lightning. Missile protection buffs. Good flying unit summoning balanced between offense and defense. Good scouting spells including far vision. Ethereal magic.
Magic was the thing that MoM got almost exactly right in terms of "fun", and imitation is the sincerest form of flattery. Further, MoM stole gratuitously many of the best elements from Magic: the Gathering, so Brad/Stardock shouldn't be shy.
Grafting this kind of magic system onto E:WoM's already nice tech-tree system would be *great.*