I don't think it'd be possible for Auron or Tidus to become the Final Aeon, though Auron would make one hell of a powerful opponent. .:edit:. of course, Jecht proves that Tidus could do it without a hitch.

My money would be on Lulu to become the Final Aeon. She volunteered almost as soon as the party discovered the truth, and she's a strong woman so she'd make a strong Final Aeon too. I wouldn't want to see her die only to become a new Sin, though, since she's got enough sadness to deal with in FFX without condemning her to that kind of existence.