Here's my take on things:

FF1 : Red Mage who would turn into the Red Wizard when he does his EX Burst thing.
FF2 : Leon or Minwu I can't really think of anyone else.
FF3 : Ignis who could alternate between the Knight, Viking, Dark Knight and Ninja because they are the coolest jobs of that game.
FF4 : My favourite character Yang who would be able to do some sweet jump kicks.
FF5 : IDK Gulaf as a Necromancer maybe? Just to add something different to the mix.
FF6 : Shadow who could pair up with Ninja Ignis and carve shit up.
FF7 : Vincent could have his own Dirge of Cerberus like battles.
FF8 : Zell becuase monks are cool.
FF9 : Vivi (I'm already getting tired of doing this).
FF10 : Auron becuase he's pretty badass and there could be some sort of conflict between him and Ject or something.
FF12 : Basche because he's a bit like Auron and can be a proper counterpart to Gabranth. Although I do like Zargabaath too so he could have a similar role to the one Golbez had in the first game.