1. The Classic: Of course he would have played "Heroes" though Changes could have been a good option as well.
2. The Recent Song: I don't think he is confident enough of his late material to sing any of it at such an event. So I guess he would done a cover version of someone else's classic instead.
3. The Interesting Collaborative: Björk would have been an option since he would have played in Japan anyway - London is taken by U2 and Mecca already leaving no room for another all-time star, the USA don't consider him as a star anyway. She could have done something from Medulla with him delivering one of the voices, a bit like Without You I'm Nothing with Placebo, Bowie as the grey eminence in the background.
Or perhaps on the african stage with Peter Gabriel - "Growing Up" would have been a hilarious idea.
Are you bad enough?