Personally I'd love a Gun Mage. I think it fits with the sort of slightly-modern-but-also-magical theme of FFXIV (look at the black mage armor concept we saw a while back to see what i mean on this, black mage armour but with goggles and other more modern accessories). Could use elemental shots, debuffs and things like that.
Would be kinda neat to shoot my friends with healing shots..
Also, Fencer and some Blue Mage variant would be my other two new classes of choice.
EDIT: HOW COULD I FORGET?! Mammet-Master. FFXIV's Puppetmaster, we already have the damn mammets! Would be awesome to be able to summon perhaps two at a time and control them from the backlines, rather than the hand-to-hand style of puppetmaster. You could customise the roles of your pets, and have a much more direct control over their actions unlike PUP's stupid maneuver system.