I haven't heard this mentioned yet, but the Treehouse stream shows there are 7 PAGES OF CHARACTERS! (the 7th one might not be full though)
(proof attached)
My absurd hopes/predictions: 99999cc, every retro track EVER, 15-20+ new tracks (at least), R.O.B., no unlock system (they just annoy me), every kart for every character, some/all karts from Tour, and MISSION MODE
I noticed a snowmobile vehicle, the new track is called Mario Bros. Circuit (b/c of the sign) and the railings aren't very prevalent on the sides of the track.