Notes:
Jackson is still scripting the thing, no matter who directs. Spielberg wouldn't be able to put in any social messages, JUST his skills as far as framing shots, getting good performances out of actors, overall visual design, etc.
Certain characters have already been cast. Hugo Weaving, Andy Serkis, and Ian McKellan almost certainly will reprise their roles, so Tim Burton can't screw them up. If Depp and Carter were cast, it wouldn't be too unreasonable (such as Gollum).
Whatever happens, Jackson is a producer for this project, and overseeing it. He won't let anybody mess it up too bad, even Michael Bay.
That being said, I'd have Danny Devito direct it. He did a fantastic job with Matilda, and I could see The Hobbit being really good if done in a similar fashion.
P.S. Thanks