Psychonauts. Let me quote a really interesting idea from the Double Fine forums:
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Well, since I doubt we're going to ever get any sort of direct sequel to Psychonauts, I think it'd be great if DF made a simple 2D point-and-click task completion game within the Psychonauts universe. I was thinking something along the lines of that old Pink Panther game, Passport to Peril, in the sense of art direction and the way you went around the world and had to complete puzzles and tasks to get places.
It really seems to fit the whole secret agent thing the Psychonauts have going on, so maybe it could also be centered around Sasha and Milla?