The notion of a live-action Pokemon movie is something that I find really interesting. Before I got into specifics though, I think they'd have to ditch the regular anime style..... No, not make it "mature", but at least make it to where it can appeal to all ages, and not just the 8-year-old demographic. Pokemon Origins did this quite well; it was still obviously kid-friendly, but it didn't feel like a glorified Saturday morning cartoon. I have tons to say on what I think a good live action movie would entail, but I guess that's for another topic.
I could see Cameron doing a good job.... If he would ever do it. Avatar boasted some incredible visuals, something I feel that could be translated to Pokemon well enough (what I mean by that is the Pokemon themselves being turned into 3D CGI). The plot would have to be completely separate from the anime, but I found my favorite parts of the anime back when I watched it were the gym battles and Pokemon League tournaments. If the plot was limited to, say, the way the tournaments are implemented in the show, I think the plot could work well enough. Although I find Cameron rather overrated in some aspects, I believe he's capable of taking what he's given and making a good narrative out of it.
Of course there's others though.... but I'm not too familiar with the world of directors in general, so my knowledge is limited I guess.
I think, if a live-action Pokemon movie were ever to be made, I'd like for it to be directed by Tim Burton. I realize that's kind of an odd decision, given his style of direction, but I actually think his unique style would fit the Pokemon series quite well. He is very good at making movies cheerful, with a more dark overall atmosphere (eg. Charlie and the Chocolate Factory), and I think that would be a good direction to take with a Pokemon film.
Of course, it would have to be done right; don't want a bunch of Oompa Loompas running around.
If Tim Burton were to direct it, however, I would not want the score to be composed by Danny Elfman. I do not think his style would work well with a Pokemon film. I am not overly familiar with movie scorers, so I can't form an opinion. I just don't think Danny Elfman would be fit for the role.