I was actually working on a system that would make spellcasting duels more exciting, but I"m not sure it would work in practice.
Basically, you would still have a normal mana pool we"re all used to that you deplete as you cast. However, you have certain spells that are sort of like a Rogue"s "finishing moves" that, when fired, start a sort of back and forth exchange between the two mages. The target of the spell would have a few seconds to react with a "counterspell" and then you would in turn be able to react with your own counterspell. All the while these counterspells are also causing damage and consuming mana for each mage. Other abilities could be used in between these to create a strategy for forcing your opponent to let the original spell resolve. If it does, you gain a huge surge of mana back and the opponent takes a large amount of damage plus whatever effect the "finisher" had attached to it.
Granted, this was being designed for a smaller multiplayer game based on spellcasters and not really an MMO, but it might be interesting to tweak it some and make running out of mana less annoying - the better you play the more mana you will get back. Right now it wouldn"t work in an MMO simply because of the large scale PVP battles, if a Mage hit you with a finisher and you were trying to fight 2 other dudes, you might not have time to switch back to him. Still, I think something similar has potential, just needs to be reworked.