From a business standpoint, this is AWESOME.
It would be like making a Batman/Spiderman/Donnie Darko movie.
Sure, it sounds like a cash grab, but you just KNOW it'll be a hit because fans from all three genres will RUN to see it.
I mean, you couldnt have set this up better if you had let DON KING announce it at a press conference... two of the most arguably well known/iconic mascots in gaming history, the faces of what was the ORIGINAL console war some 20 odd years later appearing together in a title... the only thing that could have made it better would have been to make a Super Mario Sonic Universe cross over game with huge 3d levels incorporating elements from both franchises made by Sega for Nintendo.
Then publish it on the Xbox 360.
The known universe would cease to exist shortly thereafter.