After the addition of MP regenthings became too easy for mages since the MP regen is proportional to the max MP. This would, occasionally, ends in the raising of spells' MP cost, which would be bad in my opinion.
So the idea is to match the MP cost of non-mage classes to mage classes. And, to do this, i'd change the way the "area effect" button works.
When targeting a single target (or hitting only one target while having AoE on), everything would stay as it is. But, once the spell hits 2 or more targets, the MP cost would be multiplied by the number of targets, making the MP cost similar to the case of someone being casting the spell on one by one.
Example: If you cast a cure. You spend 12 MP and heals the dude. But right now you need no skill to do this...any mage can spam cure, spending 12 MP and healing everyone in range, not even worrying if some of them actually need healing.
With this change, the correct usage of AoE commands will become important since, when he heals one target he will spend 12 MP but, when he heals 8 targets (assuming the new full party size) he will spend 96 MP.
Same works for nukes and any other spells. With the flexibility that comes with the AoE option so would come the strategy. Once the enmity is also balanced, making enmity generated receiving a multiplier based on AoE usage would also be welcome, making mages having to learn when to use a spell with area of effect on and when to use it off.
Also, this suggestion would not affect any class that is using crossclass spells but cannot use area of effect option (disciple of war) in any way, which is much better than an overall raise of MP cost of spells, that would make things much more troublesome for classes with low MP pool.
But, obviously, this suggestion only affects MP. In spells that have different kinds of cost (like sacrifice), the other costs would stay unchanged regardless of "area of effect" button being on or off.