I think Soap Shoes for Sonic are a given, and a SA2 skin for Shadow since Sonic is getting an SA1 one. Would be neat for them to get the Riders gear as well. Maybe give Classic some skins from Superstars like the LEGO one, the rabbit, and add a Christmas one for everyone post launch (it'd be especially funny to see for Shadow). Chip's Bracelet would be cool to see as well, and Sonic's favorite clothes
I'd love a Secret Rings Skin for Sonic where instead of going Super he goes Darkspine. A Movie Sonic and Movie Shadow skin seems most logical from a marketing perspective for the movie. But there's literally nothing they can't do, I mean we're getting Dreamcast Sonic and Sega Saturn Sonic for Chaos' sake. They could literally add ANYTHING. Heroes Sonic, 06, Boom, whatever.
It still hurts that Tails and Knuckles got almost nothing but Sonic and Amy got a bunch (and Sonic didn't even get a deep cut like his favorite clothes). Like, they make a Shadow onesie but not hat Knuckles or even a goggles skin for Tails? Truly a shame