The Global Mana Pool was indeed a step in the right direction.
But I think Mana could be made more useful and interesting.
Taking MOM as a reference, it would be nice to be able to invest mana in different fields, and also add constraints to the new mana pool system. The following suggestion is largely based on MOM's casting skill system and it's mana management system.
Mana Channeling Concepts
Strategic Vs. Tactical
The current system has it setup that, in Strategic mode, casters can cast as many spells as they desire, given the sufficient amount of mana in the global pool. Whereas the tactical system, only allows the caster 1 spell per turn.
In light of making it a little more interesting, it would be extremely interesting to add a value which would dictate how much mana a caster can "channel". For example, a caster with a "Channeling skill" of 50. He could only cast 50 mana points worth of spells in combat, and could only channel 50 mana points worth of global enchantments per turn.
This would help give restraint on how many global enchantments can be cast. Currently, you can have your caster teleport to each city and cast enchantments on each town if he so desires. There is no limit, only your global mana pool restricts it. And would also give more power to casters in tactical combat, should you have a caster with a high "channeling skill" he might be able to dish out more spells in combat.
Investing Mana
It would be interesting to be able to invest mana in various fields. Basing on MOM, we could have it split in 3 fields.
Mana Pool; All points against this would, obviously, contribute to your mana pool. 
Channeling/Casting Skill; Mana points invested towards this would slowly increase how much mana you can cast per turn in Strategic view, and how many combat spells in tactical. To base it on MOM formula, you had to invest the same amount as your current casting skill in order to gain an extra point.
Casting Skill 10, would cost 10 mana points to raise it to 11, then 11 points to raise it to 12, etc...
Research; In MOM you could invest your mana income towards research. They used a 1:1 ratio, depending on how much it could influence balance, it could have a different ratio for EWOM.
Closing Thoughts;
This would introduce another layer of "Guns or Butter" like the new population system introduced. One could invest all his mana towards getting faster research, where one could invest it more towards his casting skill to make sure that when he finally unlocks the big strategic spells, he won't have to wait 20 turns to cast them.
This might also allow the ability to include buildings that would increase mana per turn? Or maybe even population (like in MOM)? So tha regardless how much mana per turn you can generate, you have to invest it towards your channeling skill, acting a bit as a mana sink. I just find that currently, I build my empire and for a rather long period I don't need to use mana. And when I finally get to war, I have a ridiculous stockpile of mana, and can almost abuse the fact you dont' have any limits of casting in strategic view. If i'm not mistaken, in 1 turn you could destroy the entire landmass around an enemy town, given the range and sufficiant mana. or you could cast, as many pillar of flame, etc...
This would also allow us to have uber powerful spells, that could cost 500 mana to cast. So someone better have a high casting skill or else it would take multiple turns to complete. Preventing the user from spamming as many as he wants, which is probably something that had to be considered when designing the spells in the first place. A high mana cost isn't enough in my opinion, since you can stockpile so early on, and then just unleash hell in 1 turn when needed.
Might not sound interesting to everyone, but i'm sure this might strike a cord to some.
Anybody with permutations or their on ideas, please feel free to add
.
Cheers,
V.