Honestly, I'd be happy with any! I haven't myself played any of the games older than Awakening yet, but I'm sure even if they are ones I'm not familiar with I can learn more about them and grow to like them!
As far as older ones go, I do know of Roy and Ike thanks to Smash, so they would be kinda cool I guess!
Some favourite characters I'd really love to play as might be...
Awakening - Gaius, Aversa
Fates - Charlotte, Soleil, Dwyer, Shigure... ahh, so many!
Heroes - Alfonse!
Shadows of Valentia - Kliff!
But anyone, I'd be super thankful and hyped! :)