I don't think Sonic will show up in Mario sports, party or RPG games.
Probably a Winter Olympics game, assuming SEGA keeps the Olympics rights, which they should due to M&S's success.
I'm not expecting much else though. Sonic is getting his own Tennis game and his own RPG, and Mario sports and RPG games already sell great, so Mario/Sonic sports games or RPGs seem unlikely. And Mario Party CLEARLY doesn't need Sonic to sell well.
A tag-team style 2D platformer could be awesome, but I don't really expect that either.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.