MWEX
Explanation?
I'd think gimps are pretty easy since Sonic can reach Roy from anywhere offstage and springs are god for stopping upB like it is against Marth. Also, as soon as Roy dtilts/ftilts/fsmashes we should be able to punish the whiff. Also, whiffed shffls are punishable on landing. Somersault should be good for setting up quick gimps with the baseball slide and jc grab on DI in leads into throws offstage or uthrow (-> uair, dair, nair, bair), although other follow-ups off of somersault include nair, reverse hitbox bair, and BA (blast attack) -> fair. Dsmash should be good as well to cover tech options (i.e. dthrow near ledge it can cover all options) and it comes out quickly so it can catch poor DI. Basic Sonic stuff really.
Larger stages would be in Sonic's favour in the MU because it allows him to live longer against bair and fsmash, and not changing his game plan (knocking Roy offstage), while also giving Sonic lots of room to find his space, and dash back -> dash in to punish whiffed moves.
Just my thoughts. Doesn't cover Roy's end of the MU, and it's a little simplistic.