Modern has a few decent levels...they just end way too soon, lol. Luminous Forest is almost a good Sonic level, but it ends like three minutes earlier than it should. Modern also feels kind of off in general in this game: TBH if the whole game was the Avatar, I think the game would've been 10x better than what we got in the end, as the Avatar has the most interesting level design and gameplay gimmicks.
There's no saving Classic Sonic, though. That part of the game is just straight up bad IMO.
If they just took Generations Classic Sonic and gave him similar physics, then I think it would've been fine.
The avatar's levels were well-designed. With modern, he controlled like a dream but the levels were too short. With classic, he wasn't fun to control and thus his levels were also bad. If the controls are not in place, no level will be fun to play no matter how well-designed an individual level is.
Controls and physics are the most important part of any game. If the controls don't flow, the game will be much harder to enjoy from the start. I've held to this video game perception philosophy since early high school, and that opinion has only solidified over the years.
Granted, other factors are important too. But your game has GOT to function all-around well if you want it to be well-received.
