Two Things
First, I really liked that suggestion for Jane Seymour as the cleric. Even if she wasn't part of the party, she'd be really great for some character in this movie, good or bad.
Second, don't flame me out here, but if we were to make a D&D movie, does that necessarily mean we have to market it as D&D? Why not just market it as a fantasy movie with an ensemble cast? We'd probably be more likely to *get* that ensemble cast if we weren't connecting it to the other two movies. Of course, we leave in all the stuff that gamers would recognize, and we just tell everyone else after they spend their mulah going to see it.
Also, Wil Wheaton would be cool, but would he want to do it? I mean, would he do it even if he wanted to?