They basically have to do a 'Marvel Cinematic Universe bulid-up to Avengers', huhI would prefer that they don't push in a heckuva lot of characters until they perfect the gameplay for every. single. one (which may or may not take several games to do; I think they were trying to perfect Sonic's gameplay across Unleashed/Colours/Generations, though). They can fit Knuckles in at the moment if they speed up Werehog, fix the levels, trade Knuckles's model in for Werehog's, etc.
Tails is kind of an iffy one because I hope he doesn't have that weird ring attack. No Tornado sections, though.
As for online Chao Gardens, you should be able to get them done offline too! D:
I wonder if they could get away with having two types of Tails/Knuckles levels, one where they play it straight (more like Sonic; think 3&K), and then another where they're more in their Adventure specialty, provided Tails in the Tornado controls like Gamma or better and Knuckles is hunting something that isn't in a gigantic fucking mess