I don't know Darabont, but Kasdan would've likely been able to flesh out the basic-story, thin that is, which is now both basis and end-story of the prequels.
I'm sure he would also take the first drafts into account and use elements from that as well.
ROTS to me seems like some huge fire-cracker that in the end only goes fizzle-pop. It started out so great, that whole Palps-rescue bit had humour like the OT, action like the OT and the Dooku fight and when Anakin killed him was better than the emotion that the final duel delivered.
I am convinced Lucas is a great, very great visionary, but he is a lousy story teller. He has ideas, a vision, but he can't turn that into a coherent interesting story.
And because no-one had the guts to tell Lucas so during the PT, he obviously, and given the succes of TPM, considered his stories to be excellent.
Yes, he dropped Jar Jar due to popular request, but apart from that he rambled on, tossing in subplots that didn't really matter.
Anyway, it is a missed opportunity and in itself, indicative that Lucas didn't fully believe he could write it addequately himself.
But alas, what could've been.