If Nine Inch Nails and Depeche Mode are both actually locked down, then I think the third is absolutely going to be somebody with a mainstream edge. I would imagine that having two headlining bands with a darker sound, whose heyday was in the past (although both are still relevant and have big fanbases), means that GV will try to balance it out. There is no "formula", but I've generally found the headliners to be pretty balanced in terms of mood, fanbases, and genre.
I love The National, but them headlining is the same to me as when people suggested that My Morning Jacket were big enough to headline a couple years ago. Stone Roses aside, no radio play ever means no headliner. They are the definition of a Coachella first subheadliner, a la The Black Keys 2011, Yeah Yeah Yeahs, LCD Soundsystem, etc.
I don't think Daft Punk will be there. Just because.
NIN/Depeche/Timberlake, NIN/Depeche/deadmau5, or NIN/Depeche/Mumford sounds like the right type of mix, although I'm not saying any of those are likely either.