Alexandre Desplat would be a likely choice, he's been getting a lot of work lately. He's got range and talent to spare, but aside from Fantastic Mr. Fox (odd choice, I know) none of his stuff has blown me away.
I'm trying not to get my hopes up for David Newman. Whedon hasn't brought any of his original crew members from Serenity, Newman probably won't be the exception. He's not somebody the studios would hire for a big tentpole movie anyway