With the technology we have today in movie making, I say leave some of the characters like Donald, Goofy, and Mickey hand-drawn. A good alternative would be CG characters. But there are others, like Maleficent who would do better with a live actor. So here is a compilation of actors who I think would play the parts well. This is by no means a complete cast list, as that would take a very long time to make and think about. So this is just some of the more important characters and a few of my favorites.
Sora/Haley Joel Osment
Who better to play the part of Sora than the voice of Sora himself.
Riku/David Gallagher
Sure, we all know him as that ingeniously obnoxious kid from 7th Heaven, but he did an excellent job voicing Riku in the video game, I think it's only fitting to have him play Riku in the movie.
Kairi/Hayden Panettiere
Aside from being some nice eye candy, Hayden had voiced Kairi in the first two games before her role on Heroes. Since she already knows the character, and resembles the part, I say she should go for it.
I think having the three main character's original voice actors for the part is crucial to this would-be movie. It would also be a nice gesture towards fans of the series. Now on to the other characters.
Maleficent/Famke Janssen
We probably know her best as Jean Grey in the X-Men movie franchise, and she is an accomplished actress. She has shown that she can play all types of roles. Such as Kamala in Star Trek: The Next Generation, Xenia Onatopp in the James Bond film GoldenEye, and as Katherine in the Robert De Nero movie, Hide and Seek. I think that with some green make-up, she would be the perfect Maleficent.
Ansem the Wise (DiZ)/Christopher Lee
Once again, this is one of those situations where the original voice actor would most likely be best for the live-action role. I mean c'mon. It's Christopher Lee.
Namine/Brittany Snow
Another actor who should play their respective voice role. I tell ya, Square Enix did a great job finding voice actors for their characters. And if not Brittany Snow, I think Hayden Panettiere should also play Namine, since she and Kairi are supposed to look alike in facial features.
Roxas/Jesse McCartney
While it pains me to no end to say this, I think Jesse McCartney would be good for the role. He was the original voice actor of Roxas in the game. He actually did a really good job in getting emotion out of a character that is supposedly emotionless. I applaud him for that. He looks the part, and he can act the part. Regardless of my personal opinion of him, he would be perfect for the role.
Xehanort, Xehanort's Heartless ("Ansem") and Xemnas/James Franco
Franco has proven himself to being a very versatile actor. He's good at getting deep down into a character and making it real. Whether he's doing drama or comedy, this guy can pull it off.
From this point on, I'm not going to explain my reasoning for picking an actor unless I feel I have to. There are just too many to explain :P
Xigbar/Jeremy Irons
Xaldin/Hugh Jackman
Saïx/Ralph Fiennes
Saïx is a character that needs to be calm and collected that can explode at a moment's notice. This guy can do it. He's a classically trained Shakespearean actor, was in the movie Schindler's List as Amon Göth, is going to play Hades in the upcoming Clash of the Titans movie, and is probably immortalized by playing Lord Voldemort in the Harry Potter movie franchise.
Demyx/ Unknown Actor
I say go for an unknown with some talent
Luxord/John C. McGinley
Axel/Justin Long
While not the most physical or athletic of actors, he definitely has the attitude and smart mouth to play Axel. Besides, stunt doubles aren't hard to come by.
Auron/Bruce Willis
Yuffie/Brenda Song
I know she usually works for Disney, so this would be right up her alley :P But seriously, it would also be a way for her to expand her acting career.
Leon (Squall Leonheart)/Jon Foo
Sephiroth/ Unknown
For Sephiroth I say go with an unknown. Unless you guys/girls can think of anyone.
Same goes for Cloud and Tifa