@decra: it needs to be attached to some external game, yeah, but I honestly don't see why it needs to specifically be a Sonic game. without any external game all you'd really have is glorified versions of the Tiny Chao Garden games for the VMU and GBA, which are both not very good or stuff like Duck Life or Chao Resort Island which are kinda alright but have really flat progression curves.

the chao garden only works because it's a hyper-expanded Tamagotchi experience that functions as an external side-progression system attached to another game. but, really you could attach that to any game. you just need a satisfying way for a player to grind rings and small animals/chaos drives, and there are definitely ways you could do that without just collecting them in a stage like in Adventure 1/2. and, if they had this Chao Garden detached from any one game you could do stuff like give special cosmetic unlocks for new SEGA games you play, or really any media; movies, comics, whatever.