What would most surprise and excite me would be Kid Icarus for Wii or a new 2D Metroid for DS. If it's really a big, "famous" series, if we look at Nintendo's biggest franchises, the ones we just know will appear on Wii sooner or later, I'd wager it's either F-Zero or Mario Kart.
But the realist in me is expecting it to be a DS game in an obscure, Japan-only series that will never see a U.S. release.
Edit: I remember another famous series that's a natural for the Wii: Pilotwings. I'll add that to the fantasy column.