There seems to be a lot of misinformation floating around the web regarding the RHCP. Yes Anthony Kiedis said they were taking a break (not disbanding as NME sensationalized) for about a year. But their last show was at Reading/ Leeds in August 2007. And back in February of this year Chad Smith mentioned they may be ready to work on material in the fall. The schedule seems to work out okay so don't count them out just yet.
I think RHCP are a band that were a lot more fun when their studio stuff sort of sucked. A lot of people I know who saw them back in the late 80s, early 90s rave about them. At the same time, everyone I've talked to post-Californication seems to be fairly lukewarm about their live shows.
Oh, and for the question at hand, I think Muse, Rage Against the Machine and Beastie Boys are among the most likely headliners with Zeppelin, NIN and Coldplay all being slightly less likely possibilities. I would love to see Neil Young, Paul Simon or Prince thrown in as possibilities. And of course, the return of Radiohead would always be welcome.