I would like to see a chao garden with online play but a few things would have to be balanced out and adjusted, stuff like:
Animals, you find these playing the main game's levels, how would you change the method of finding them?
power cores, another item from bots in the levels (or just using animals like SA1) would have to be earned in a certain way too.
Chao stats and shit, would need to be rebalanced so people with top end chao's could not abuse one method to win all the time.
Also to stop things being boring they would have to add more circuits or some random layout generator for races, the course in SA1 was better then SA2's also.
I guess the ability to visit friend's chao gardens would be nice to let your chao play with theirs, also that would work well with the chao breeding part to make those rare chao.