There was a lot of talk about Mario at this year's GDC — Game Developer's Conference — including the revelation that Super Mario 3D Land could have had skateboarding. Koichi Hayashida, a leading figure in recent Mario titles, revealed that particular piece of information when on stage, but in an interview with GameTrailers he turned his focus to ideas that didn't make the cut in Super Mario Galaxy 2.
When quizzed about potential Mario power-ups inspired by Nintendo characters from other franchises, Mr Hayashida instead confirmed that he'd proposed Donkey Kong and Pikmin cameos in the Wii sequel.
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.
Shigeru Miyamoto is well known for insisting on function and purpose in features and cameos, so the reasoning for the rejection of the Pikmin proposal is perhaps not surprising.
In the Pikmin games, you'll notice that all the Pikmin are very thin — tall and narrow for their frame. Mario enemies tend to be on the short and squat side and there's a functional reason for that. They're easier to stomp on. They're lower so you can get above them and broader so you can land on them. In that sense, Pikmin are just not well suited for the Mario universe.
With Donkey Kong also missing out, we had to content ourselves with the appearance of Yoshi and that wonderful flutter jump. What cameos, no matter how unrealistic, would you like to see in a Mario title?