"One of the early proposals that we discussed for Super Mario Galaxy 2 was the possibility of including characters from other [Nintendo] franchises. For example, you might have Donkey Kong or Pikmin show up. We presented this to Mr. Miyamoto, but he came down pretty hard, saying that there has to be a functional reason to include characters of a certain type in a game. He went to specify precisely why the Pikmin wouldn't work." - director Koichi Hayashida

I have to agree with Miyamoto on this one. Those cameos just don't seem like they'd make much sense. Glad he stepped in to shoot down that idea.