I think the only ones that really make sense are prequels to characters with more adventurous backstories. IMO any sort of sequel is just stupid. In any JRPG the characters have become so god slaying powerful that any attempts to have them start over from the beginning of an adventure just feel forced.

Kain is an interesting character that I would like to see more of as he is off journeying on his own, but I can't see that making a good game.

Having a game showing what Auron was up to in between Jecht's defeat of Sin and the start of FF10 would be interesting. (I can't believe I said a return to the FF10 world would be interesting.)

Balthier and Shadow are both great suggestions, they obviously have great adventures prior to their in game appearance.

If you wanted to you could take Orlandu and make a wicked game about the 50 Years War.