I honestly can't see them ever making a Live-Action Dragonball movie that I'd be satisfied with...
But since this is completely hypothetical and could only ever happen in your dreams anyways, there would be some people at one point in time, I wouldn't mind being cast as certain characters; judging predominantly by appearances. For the Saiyan Arc:
Bill Goldberg as Nappa
Well, the picture probably speaks for itself.
Ziyi Zhang as Chichi
She's starred in a number of martial arts films, and when shown to be angry, I think she gives off enough of that intimidation/craziness that you'd expect from Chichi.
Pat Morita (RIP) as Master Roshi
Mr. Miyagi from
The Karate Kid, he would've fit the role perfectly, IMO.
Danny DeVito as North Kaio
They are both short, stocky guys who also have a comedic side.
I'll probably post more later...