I say no. I'm a big Mario and Sonic fan, however I find the two worlds are far too different to both be in the same adventure games.
Also the Mario games usually have little talking in the stories etc, whereas Sonic features alot of voice acting (and ever since 4kids took over that voice acting has become mediocre). However depending on the style of gameplay, I suppose it could work however I doubt it would happen.
It's more likely we'll see another spinoff with Mario and Sonic than we'll see an adventure title.