The following ideas are going to annoy some people who are hardcore fans of the show. I think change is good and I think the main problem with Dragonball Evolution wasn't the fact that it made changes but the fact that it changed too much. The key to a good adaptation is to make it original while keeping the heart and feelings behind it in tact. That is also the key to mass appeal, showing fans what they loved but opening it up for a broader audience. Here's some ideas to take into account for a DBZ adaptation:
Focus on DBZ
Dragon Ball is a fantastic show but most people know Dragon Ball Z. Dragon Ball is way too long and expansive to try to tell it's stories before we finally get to what people know best. Part of the problem with Dragon Ball Evolution was they skipped nearly all of the build up to the eventual Piccolo fight, the Red Ribbon Army, Emperor Pilaf, how Goku really met Chichi, even the flying nimbus was excluded.
The first 10 to 15 minutes of the movie should cover the entirety of Raditz. Have Gohan partially narrating what happened with Raditz and how his father died like this:
I had just met my father's friends when everything went wrong. Before I knew it I was kidnapped by Raditz, a strange man who would turn out to be my uncle. He revealed to me that my father is of an alien warrior race known as the Saiyans. My father and his worst enemy Mr. Piccolo rescued me from Raditz and my father gave up his life to do so. We soon learned that more Saiyans are on their way here and we only have a year to prepare for them. Mr. Piccolo took me from my home in order to train me to be a warrior. I have to make my dad proud and keep my planet safe.
Not my best writing but it works as a jumping off point. It hits all the key points that one would need to know, except for the Dragonballs which could be edited in or mentioned later. Still you would be able to show an epic fight of Goku and Piccolo vs Raditz by putting spaces in between the narration and showing the fight but you don't have to waste time on the build up and things on a D level character like Raditz. Anyone who has watched the show knows the story and anyone who hasn't doesn't care about Raditz anyway. He can come up in some small way later.
Skip the snakeway
It's a 2+ hour movie we don't have the time to follow the snake way or the movie will be called "Goku's excellent snakeway trip". Snakeway should be a sidenote of how he gets to King Kai's place. Although, about Raditz coming back later. It'd be cool if Raditz caught Goku off guard as he was flying back and grabbed him pulling him down to Hell while Goku fights to escape. It would add extra dimension to why it takes Goku a long time to get to his friends.
Focus on Training
The majority of the movie should be training but it should be good and interesting training. Show the Z-fighters training with Mr. Popo on the lookout and have them facing their worst fears. Show Gohan and Piccolo as he trains him to survive, fight and fly while also displaying Piccolo growing softer as a surrogate father. Scrap the Goku bonking Gregory training but do keep the chasing Bubbles training for comedic effect. Also have Goku actually training the Kaioken and Spirit Bomb but failing a few times.
Keep Mr. Popo
I don't care if he is a racist caricature of a black person keep him anyway. Mr. Popo is awesome.
Make Use of Minor or Nonspeaking Roles
Just because people are there doesn't mean we have to focus on them but it also doesn't mean they should be excluded (Krillin). Bulma, Master Roshi and Turtle, Puar and Oolong, Kami and Korin are all non essential characters to the direct story that should still be included in a minor way to give the film some authentication. Also let's not make the important roles as minor, make the Z-Fighters relevant and strong, except Yamcha go ahead and kill him with a Saibaman again.
Save the Villains for Later
Don't show Vegeta and Nappa until the last half of the film. Let them be a looming threat. This would be like a natural disaster flick; you wouldn't give the asteroid from Armageddon pathos. You know who Vegeta is, you know why Vegeta is there, you don't need to see his journey to Earth. That kind of thing is for sequels. It's like Vegeta is Galactus and Raditz was his herald.
Update and Alter It
Modernize but don't go crazy. Give the movie real stakes have the Saiyans come down and start by destroying some people Androids style. The big explosion was too impersonal, as was the fight against the army. Add some real stakes. Instead of fighting in yet another wasteland have them fight in the middle of a destroyed town. Don't worry about making it less than 2 hours, push that time a little. Feel free to really explore the Z Fighters. Also I think it wouldn't hurt to break off from the story some and maybe add some characters. Maybe in a later movie add Pikkon to movie canon, or have Future Trunks travel to Namek to get a new guardian of Earth and wish Future Gohan back.
I don't know the perfect way to make a Dragonball Z movie or what every fan wants to see. I'm not a super dedicated fan myself, though I do own all the orange brick DVD boxsets except for the Saiyan Saga. Some people might really love Raditz and want him to have his own movie but I think what people really care about is Vegeta. That's why it's best to have the movie structured in a way that shows off the Z-fighters we love but fully shows the pure evil Prince of Saiyans from the Saiyan Saga, who killed his own underling. DBZ does have a seriously dedicated fanbase though so it might not be possible to make a DBZ movie to appease the fans. Also Patton Oswalt for King Kai.
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