I like to think FFXIV will be a polar opposite of FFXI's engine for magic. You can't have the exact same system for a fresh game. So here are some of my theories are listed below.
Because FFXIV won't have a level system, what if the spells duration/effect were adjusted according to your "growth" chart. For example, Protect/Shell will reduce physical or magical defense by 50% like most FF games, but the duration will be quite short at the beginning. As your mage's magic abilities grow(level up), you'll learn new spells and abilities. So of course it won't be as feasible to recast Pro/Shell as often then. Apply the same theory to haste. The mana costs, recast, duration, and potency would all improve based on your skill. Kinda like a skill up in FFXI. Mage's enhancing skill rises 0.1 points.
The more destructive your casting style, the more your MP pool would increase along with your magic attack rating. Lost spells like Meteor and Ultima would only return if you casting style was so reckless, those spells became available due to desperation. Imagine a system where your behavior would cause your character to summon their inner power like a customizable 2hr in FFXI. My inspiration for the idea is taken from none other than FFIV's mage Tellah. Granted the use of Meteor by Tellah was scripted, but the potential is there. If your mana pool was exhausted, you would give up your HP in exchange borrowing for the extra mana required to cast the nuke. Consider that feature a real time Convert casting cycle.
Comments, questions, feedback?