Green Hill or Cherub Island (just a miniature Angel Island. I don't like too many walk-off edges, and this'd explain why it's floating).
Either way, they HAVE to have a loop-de-loop. There are two things I thinks of when I think Sonic: Loop-de-loops and springs. Springs have already appeared in Sonic's moveset, so they're exempt, but we NEED a loop-de-loop. The physics would be rather easy, actually: Do it just like in the 2D games. The graphics would be much more of a problem, but they could do it. Even if they can't, I don't care. The loop-de-loop is a MUST, even if it looks awkward.
Other motifs they should have (but not as REQUIRED as a loop-de-loop) are spikes, crumbling cliffs, floating/moving platforms, dash pads, spring boards, and ramps.
If there's a second stage, I don't care, as long as the first one has a loop-de-loop (or, they could construct additional pylons). There are several possibilities: ARK, Death Egg, Egg Carrier, Station Square, City Escape...
I do REALLY want a pinball stage, but it should be series-neutral, only really representing, well, PINBALL. After all, Mario, Pokémon, and Metroid have had pinball games, too. The bumper made specific reference to pinball, so that gets my hopes up.
LOOP-DE-LOOP FOR BRAWL!