Blue Mage would be awesome, always been my favorite class. I think it'd be interesting to do a Wrath-like mechanic. Doing physical combos (Vorpal Blade! Expiacion! Sanguine Blade maybe?) builds up whatever blue magic stack, when you get to five you can blow it on a spell. Maybe five lets you do big ones (Heartstopper? Bad Breath?), while fewer are necessary for healing/support options. Put cooldowns on them to make White Wind more like Second Wind than Cure in terms of frequency, so you aren't suddenly a healer class.

A huge amount of the fun of BLU in FFXI was hunting down the spells (and yes, I had them all, and yes, I fully acknowledge my rose colored glasses), and I don't really see that happening in our current FFXIV state. It'd be learning spells from job quests still, just with the veneer of "hunting down monsters".

I'd also love to see Time Mage. Comet, Demi, Meteor, Double, Triple, Old, all awesome spells.