RUMOR: More info on Nintendo x SEGA - 3rd Sonic game is a racing game staring Nintendo crew, Deal may extend to other non Sonic SEGA games and expect Sonic and Bayonetta in Smash.

[Gamerace]

We'll see if this turns out true but a Mario guest star in Sonic Riders makes a lot more sense than a Mario/Sonic Kart.
Mario would boost Sonic Riders sales and both companies profit as a result. A Sega produced Mario/Sonic Kart would only benefit Sega as it wouldn't boost Mario Kart sales at all.

However if Mario Kart 8 misses this holiday season, then it's going to suck to be WiiU.