In terms of other bands or artists I still am hoping for maybe...
List of dream bands/artists that would be cool to see somwhere in the line up (in no order in particular): AC/DC, David Bowie, Prince (but wouldn't Prince, Bowie and AC/DC be headliners though?), Flight of the Conchords (that's right I said it), JAMC, Motorhead, Underworld (for the dance tent), Chemical Brothers (might as well say them for the dance tent too), The Mighty Mighty Bosstones (since they've been doing a few reunion gigs here and there), Madonna (she does have a new album coming out soon), Portishead, The Cult, Love and Rockets, ZZ Top, Dwight Yoakum, Willie Nelson and Blur.
Realistically what I would like to see (as in these bands seem more possible than not): Oasis, The Verve, Vampire Weekend, QOFSTA or EODM (Either way Josh is going to be there), Muse, MGMT, She Wants Revenge, Be Your Own Pet, Kate Nash, The Black Keys, The Raveonettes or The Kills, Cat Power, Hot Chip (dance tent), The Cool Kids or Lupe Fiasco (it makes sense with Kanye being there), Spank Rock, Mars Volta, Against Me!, The Dirtbombs, Beach House (considering that they are from Baltimore), Mudhoney, Datarock (dance tent), Diplo (dance tent as well), Flogging Molly, Carbon/Silicon, The Raconteurs, Mark Ronson (most likely dance tent), Danny Tenaglia (dance tent again), Sasha and Digweed (dance tent), Gogol Bordello, Girl Talk (He was so much fun last year in the dance tent), The Hold Steady, Rancid and The Breeders.
Honestly if I get a combination of any of the mentioned above (both lists), I think I'd be pretty happy. Hell, I might even go to both days if I can pull it off!
ps- I really hope that Metallica aren't being booked. That would be like the snafu back in 1996 with them on Lollapalooza. But if they are, I just hope to avoid them.