I reckon Bay might call back Linkin Park, since they've done the last two movies and have made very successful singles. If not then maybe some band that has the new, contemporary clear-cut rock sound similar LP. I'm thinking a possible replacement would be Pendulum. Or even Disturbed if they make a song TF-song that sounded as futuristic-like 'Inside the Fire'.
Metallica, eh, I don't hate Metallica but their metal style I believe is more suitable for less sci-fi action movies (examples like Mission Impossible 2 with 'I Disappear' and Zombieland's with 'For Whom the Bell Tolls').
And I'd doubt that Metallica would want to make a song for Transformers, would it even be something that the band members would like?