I'll add a "vote" for Super Mario 64 2. If I recall there was a screenshot of a split-screen mode with Mario and Luigi. What a shame that never saw the light of day, or even something similar in the Galaxy games.
Could you mean this picture?
I've seen it before when SM64 2 was refereed too, but it's a fake. That level is in SM64. If there is a real pic, i'd LOVE to see it.
Yes, to this day do I wait for a game with the features they proposed.