In terms of game mechanics, I think the Legends style could translate perfectly into 3D, with some tweaks of course. I say that because, when I watch footage of Sonic Lost World, I do think of Origins and Legends, right down to the fact there is a run button. And if Sonic's speeds can work in 3D along with the parkour moves, so can Rayman's, seeing as he doesn't go quite as fast as Sonic.
Throw in some moves from Rayman 2, like the purple lum swings and the swimming mechanics, and maybe something like the power-ups in Rayman 3, then I think we'd have a winner.
Though a game like that would probably require fine camera work.
And regarding online play in a new 2D game, I understand Ancel's reasoning for not including online play in Legends, and he has a point, but I think he's forgetting that people who are passionate about his series may want to play with each other, but can't because they are too far away from each other. I rarely get to play co-op in Legends, and only did so once or twice in Origins. The only people who like the game that I know don't have much time to play it with me (granted, I don't have much time to play it right now either, but mid-term is just a week away).
Though there is also the issue of lag. Online play in a game like Legends would have to be almost completely lag free.
