While Dragon Ball Evolution's biggest problems were in story and adaptation, it also had a significant problem in the casting department. The film managed to cast Asian actors in a number of roles, including Chi-Chi, Mai, Master Roshi, Grandpa Gohan and Yamcha, but the main characters -- Goku, King Piccolo and Bulma -- were played by white actors Justin Chatwin, Emmy Rossum and James Marsters. This was problematic, as a beloved Japanese property had been significantly whitewashed, an issue that could be fixed in a potential future adaptation of the franchise.
The Saiyans in a Dragon Ball Z film should be played by Asian actors, with a widely diverse cast making up the earthlings that support Goku. Though this proposal may not be perfect, it is far better than how Evolution handled its casting. If a new Dragon Ball live-action movie ever gets greenlit, special care should be taken to include a diverse cast, one that makes sure to recognize the Japanese origin of the series.
Diverse casting, streamlining sagas and starting from Dragon Ball Z are all relatively easy choices to make, but style is what will make a live-action Dragon Ball Z film franchise work. If Evolution deserves any praise, it's that it made some style choices (bad ones), but it was bold enough to take some liberties.
Again, these choices didn't work out in the film's favor, but it was on the right path, though it went too far in the gritty realism direction. Whatever style choices are made throughout the production of a live-action Dragon Ball Z film, both in terms of visual design and story adaptation, should be bold.
In other words, don't just copy and paste from animation to live-action like Disney's remakes have been doing. Instead, the makers of these potential films should stylize everything. Don't be afraid to use practical makeup for aliens, have fun with the costume and set design, make Super Saiyan hair more like fiery energy than actual hair.
The specifics don't matter, it should just be clear that the films aren't a cash in, and that this version of the beloved anime series could only be done in a live-action/CGI format. Instead of being an adaptation, it should be more of a reimagining, made by fans who grew up with Dragon Ball Z and have been influenced by it throughout their careers.
This is what could make a live-action Dragon Ball Z film work, and even if fans don't want it to happen, bold choices and smart streamlining can prove them wrong. It's not like the world needs a live-action Dragon Ball Z film franchise, but if it has to happen, can't we ensure it comes out as epic as possible?