Freya would be a good one. She has a back story, but she's a side enough character in the game that her own story would be fresh.

Though I like Auron, the problem with him having a game (with Jecht and Braska) is that their story was such a focus of FFX that any prequel would have to go through severe retcon to be interesting or fresh. And you can't have a sequel with him for...obvious reasons.