So this is just a bunch of random thoughts, based mostly on who is releasing albums next year and who could possibly be reuniting and who are big buzz bands now that haven't played Coachella yet.
R.E.M. - They've got a new album coming out in the Spring and have never played Coachella. They seem like a perfect fit as one of the headliners.
The B-52s - They're not headliner material, but could definitely find themselves on the second line of the poster. They're releasing their first album since the early 90s in February.
Morrissey - He's got a greatest hits album coming out in the Spring and a new album coming out in the summer/fall, and he's already said there will be a massive tour starting in the Spring. Like the B-52s, he's not a headliner, but a very strong second-line act.
Dave Gahan - He just released an album a few weeks ago, and though it'll be about 6 months between the album's release and Coachella, it's still possible. I think it really depends if he's still touring in support of it in April.
Other upcoming album releases:
The Mars Volta
British Sea Power
Hot Chip (I wonder if they'd play two straight years)
Bands Getting Buzzed About Who Didn't Play Last Year:
Simian Mobile Disco - A perfect fit for the Sahara.
The Go! Team
Iron & Wine
Les Savy Fav
The Shout Out Louds
Tegan & Sara
The Stone Roses