let's talk limit breaks/desperation attacks

That is quite impossible (to my knowledge). The best you could do is Get rid of Lore (sorry Strago) and turn it into Limit. Then have each character have a weapon, armor or relic that only they can equip that is the only thing that teaches that spell (Limit).

Now the final and most crucial (and tedious) part: Since it is still magic, this means the use of mp will be crucial. Something simple could counteract this such as:

-lower the MAX MP to something like 500 or so (even 300 would be good)
-Don't give mages Limits (they'll need a higher MAX MP)
-Get rid of most MP restoring items (like all the ethers) and change some of the items that restore hp and mp. So basically make to where only Tents and Inns restore MP.
-In order to restore that lost Mp, you could have a set of weapons which will drain it from enemies. (Mages would probably only use these)
-With that you could have your Limit people dual wield the mp draining weapons and their normal weapons.

All you have to do is consider those with Limit. Imagine their MP being a type of gauge. Once it gets to a certain point you can use of their Limits if you've taught them any.

This is just a suggestion really, take of it what you will. ^-^