With a port of SA2 to the 360 there should be some ample spare power and with that I think the game could have:
Revamped models but instead of the slightly improved Battle models sega could pull their finger out and try to use the Unleashed models or style.
Rock solid 60fps in all areas, though the dreamcast does this very well already
Multi online using the Battle setup.
Then other things like achievements for beating the game as good, bad and final ending, perfect super final boss fight etc.
Oh and of course the chao garden with online races and breeding.
It would be rather awesome