I remember in an interview a while back that it was mentioned that U2 was really planning something "big" for this 25th anniversary.
I personally think it would be awesome if they re-recorded some of their early work such as 11 O'Clock..., I Will Follow, Out Of Control or whatever, and made a sort of EP of it and re-releasing a song as a single. It'd be awesome to bring an old U2 classic to a new generation(lets face it, most people outside of U2 fans know much of their early work).
I'm highly doubt this would happen, but it'd sure be awesome. In any case, I don't see any of the ideas mentioned so far in the thread, especially simply don't the predictible b-side thing, as being "big" as U2 described it.