No Zimmer and no Horner. Horner does exceptionally well at dramas but it's been quite a while since I've heard a really good action/adventure score from him... last one I heard was The Rocketeer and that was released in 1991.
And I don't want Zimmer. He's a good composer and all, but for the past few years he and his RC team (such as Steve Jablonsky) have pretty much dominated the summer blockbuster sound in terms of scoring, and I'd like a return to the purely orchestral approach to superheroes. I'm sick of the synthesizers and electronics put over a repetitive stream of strings and brass. John Powell has developed quite a bit since graduating from RC, and his X-Men 3 score was impressive. It sounded nothing like Zimmer would do. Plus, Zimmer has the solo Batman franchise to himself and James Newton Howard. He doesn't need to add JL to his resume.
If Warners insists on a Zimmer-like sound for JL, Miller can hire one of Zimmer's collaborators like Nick Glennie-Smith, Steve Jablonsky or Henning Loehner -- and they're cheaper, too.
But I wouldn't say no to Howard Shore or Marco Beltrami. They're pretty good choices in terms of delivering big, epic action scores.