I just picture him being CGI. I think he's the perfect choice because as we've seen in some of his films he can be goofy & funny (that's actually the way he is in real life) & he can look really badass & serious when needed. He could actually portray either Goku or Vegeta.
How come you want all the aliens to be CGI? Haha. CGI should only be used when make up is insufficient to handle the job. CGI is acceptable for creatures like the Kraken or Davey Jones from PoC, but characters like Nameks and Freeza are better done with make up because they have no eccentricities which can only be made real with Computer generated Images. Perhaps if Nameks had face tentacles or some sort of odd body structure like Jabba the hutt CGI would be the only way to go...
However, Nameks and Freeza are what you would call "Humanoid" aliens. Thus, they can be portrayed by human actors wearing make up.
Example. Kit Fisto, general human shape, 2 eyes, 2 arms, 2 legs. Easy.
Example #2. Sebulba, his body is odd and can't be done by a normal human being. He walks on his arms, his legs bend weird, and his face is so far from human that a prosthetic mask would make him look like a ninja turtle.
CGI costs an incredible amount of money to produce. The only reason CGI characters are used in Star Wars at all are for 2 reasons only. #1 George Lucas has more money than god and doesn't care how much it cost to make. #2 Those Characters that are CGI only appear for small amount of time unless extremely pivotal to the story.
Nameks and Freeza along with the rest of his army are ALL humanoid aliens, thus they can be easily done with make up. Dragonball Z will have spend its money on building space ships and CGI laser blasts, they shouldn't waste what little money they have on something that can be done far cheaper. Jar Jar binks must have cost like 200k while a piccolo done with make up will cost about 10k maybe less. Thats money that could be spent on a GIANT MONKEY, which I would much rather see.
See below, 3 humanoid aliens all standing together interacting with live actors.
This is significantly cheaper than CGI, not to mention, It's more impressive to see living beings to kung fu than to watch a computer try to simulate how a human being looks while doing kung fu, which would be the case in a DBZ movie. 
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