I played Super Mario 64 (and plan to get it off Wii virtual console, already got the points card to get it and Ocarina of Time - wish they had Goldeneye) and I love it, I just like playing coop games. Also IMO, Mario 64 had the best music out of the entire series. Actually I don't have two controllers yet for Galaxy (although I do now)... thing is I am about 300 miles away from my Wii and have been so for last couple of weeks due to exams.
Sorry, this all got carried away. I just don't see the harm in a multiplayer game because it would be unique. However, I don't think it should be a platform Mario, I think it should fall in the shadow (since it is an experiment) of a major Mario. We can have more then one Mario games on a platform (Mario 1-3 and LL on NES for example).