To be fair, there are a fair few features from old mods such as ShufflarB2 and XSRB2 which I wouldn't mind being ported in some way, be it through modification or another method:
> Slufflar's Mario mode in particular, being more Mario-oriented as opposed to just "Sonic in a Mario world" (ie: No coin loss on hit, mushroom for more health, no spindash, etc)
> Shufflar's Ring Collection mode was stupid fun. Basically a kind of Capture The Flag, except with rings. Plus, colourful ring rain.
> XSRB2's Sonic Golf was an interesting spin (heh) on the classic golf formula. A fun thing to mess around with, would be nice to see it expanded a bit.
> XSRB2's Shards is something I would actually like to see ported to Vanilla in some way (though perhaps involving the Chaos Emeralds?) - Plus, a gamemode like that makes rail rings nowhere near as useful - if you're sniping someone from afar.. how are you supposed to take their Shards? You'd score no points. It's useless.
> XSRB2's death on character change. I really wish this was a thing in the vanilla game, or at least a way to prevent people from switching mid-game in gamemodes like co-op.