It seems Mario and Yoshi almost had to share that Super Mario Galaxy 2 spotlight! In an interview with GameTrailers, director Koichi Hayashida revealed that during the development of Super Mario Galaxy 2, cameos from other Nintendo franchises were at one point a possibility:
He goes on to say that they presented the idea to Miyamoto, who came down on the idea swiftly, saying the inclusion would need a functional purpose. He also cited the Pikmin's design:
Well, fair enough Miyamoto. A Donkey Kong cameo could have worked, but at least we'll always have Super Smash Bros. if we need some crossover fun, right?
Miyamoto vetoed Pikmin cameo in Super Mario Galaxy 2 [Official Nintendo Magazine UK]