First off I liked Dragonball Evolution I thought it was OK but not great it had a lot of problems and did not follow the source material barely at all. I want a DBZ movie that follows the material and is not hollywoodized by people who know jack squat about the cartoon or the manga.
Goku-Matt Lanter
I think that this guy looks like he should play a super hero of some sort and I can see him being a naive, young defender of the Earth.
Piccolo-Taye Diggs
Diggs is a cool actor and he would pull off this role well Piccolo is my favorite DBZ character and this guy would be pretty cool and the role.
Gohan-Colin Ford
I think Ford would be a good teen Gohan but I am not sure who would be a young Gohan they should get some nobody kid to play a young Gohan.Ford has been on Supernatural if you don't know who he is.
Yamcha-Ryan Kwanten
I don't know much about Kwanten I haven't really watched True Blood but I have heard good things about this guy and he has a good look for Yamcha.
Vegeta-Christian Kane
Kane is good at playing a tough guy and Vegeta is the definition of a tough guy.
Master Roshi-Ken Watanabe
Do I have to say anything he is awesome.
Bulma-Maggie Grace
Raditz-Taylor Kitsch
Kitsch is a good actor and would pull off a villain well and he has the look of Raditz.
Nappa-Steve Austin
I don't need to say anything he is perfect for this role just look at him.
Frieza-Andy Serkis
With this cast I think this could make a good Dragonball Z adaptation. Stay around I will make another cast like this for the other characters of the DBZ universe. Sorry there is no Krillin I had no idea who to pick for pick for him.
Enjoy.