Dragon ball is a well known franchise, undoubtedly a worldwide success, so i think it's not really necessary do movies based on the original DB series before adapting DBZ. In my opinion a small introduction during the opeening credits, narrated by a character like 'Muten Roshi' or 'Kami sama', telling the adventures of Goku's childhood and youth and explaining a little about the world, would be enough to situate the common viewer who never watched DB. One of the things openly discussed about anime adaptations is the ethnicity of the characters. I think a good way to follow is use the real world as base. So a guy like Krillin could be considered as Asian, and a girl like Bulma who has blue hair and a blonde mother, could be a caucasian. The same can be applied to the Saiyans.
In the series they're a race really similar to the humans, so in the movie they could have different ethnicities too. Guys like Nappa could be caucasian and Vegeta asian.
So, the movie starts showing Raditz's spaceship traveling through space and approaching to the Earth. So, we are introduced to Goku, Chichi and Gohan. Goku and his son are leaving to meet his friends, after 5 years of not seeing each other. They all meet at the Kame's House, soon Raditz arrive, telling about goku's origin, kidnapping gohan, everything like the anime. Goku and Picollo meets up in the way, and here we have a little recap about their history through the dialogue of the two.
Both with Raditz, Gohan show a small part of his hidden powers, about to die Raditz tell about the other 2 Sayans coming to Earth, Goku dies, just like in the anime. Here begins the little changes. Goku's training on the other world will be the main focus.
The travel down the Snake Way will be part of King Kai's training, and all the challenges with the monkey Bubbles and the cricket Gregory
will be cut. Gohan's training with Piccolo could put more focus on their relantionship than in the training itself.
The Training of the other Z fighters with Kami, will serve to show more about the Saiyans hystory, by that illusion/revival of memories that
he used in the show.
Vegeta and Nappa arrives, The Z fighters try to stop the Saiyans and gain some time until Goku make his way back from King Kai's world
to be ressurected by Bulma and Roshi that will be on stand by with the Dragon Balls. Goku arrives, and start the epic battle with Vegeta.
In the end, Goku let Vegeta live, and Bulma, Krillin and Gohan begins the preparations to travel to Namek.
We could have a post-credits scene with the Frieza being informed about the Dragon Balls, the Saiyan from Earth and Planet Namek.
CAST
Son Goku/Kakarrot - Hiraoka Yuta
Let's face it, Goku isn't othe deepest character ever created, but this is what makes him especial. To play the role you need someone charismatic enough to show all the innocence, kindness, honesty, funny and determination of the hero. Hiraoka is an actor with a lot of experience in TV Dramas. He made some movies in Japan, and won the 'Best Newcomer' prize at the Japan Academy Prize in 2005 for his film debut. He played some funny characters in his carreer, and has this
good guy face indispensable to plays Goku.
Vegeta - Jay Chou To play the role of, probably, the most complex character in the franchise, you need someone very talented and capable. Jay Chou is a multi-talented artist. He's a musician, singer, songwriter, music and film producer, actor and director. After seeing him in 'The Viral Factor', i realized he can be an awesome Vegeta.
Piccolo - Adewale Akinnuoye-Agbaje Piccolo is another strong character from Dragon Ball. Adewale Akinnuoye-Agbaje, best known for his roles as Lock-Nah in The Mummy Returns, Nykwana Wombosi in The Bourne Identity, Mr. Eko on Lost, is the right guy for this role.
Son Gohan - Suzuki Fuku Gohan had always been an annoying character to me. Maybe, this promising japanese child actor can make him more likable.
Krillin- Justin Chon The best friend of Goku, Krillin had always been one of my favorite characters. Sometimes taking the spot as comic relief, is a very likable character. Having some teenage shows in his curriculum, Justin is probably best know by his role in the Twilight Saga (blergh!). This could be his chance to shine (not like Edward lol).
Chiaotzu - CGI character, voiced by someone Chiaotzu...is Chiaotzu, you know? Kind useless in Dragon Ball Z, he would be just a CGI fanservice to the fans.
TienShinhan - Byung-hun Lee Even not being used so much in DBZ, Tien is pretty bad ass character, who actually faced villains like Cell and Buu, even not having 1/3 of their power. Lee is an awesome actor, with awesome movies like "A Bittersweet Life" and "I Saw the Devil" in his curriculum. So, i think he could be an awesome Tien.
Bulma - Laura Ramsey Bulma is the strong woman from the series. All the adventure basically started with her, and her serach for the dragon balls. She's the one who made the Pridefull Prince of the Sayans, build a life and a family on Earth, and she's also the ecchi source
from Dragon Ball, with all the panties jokes. Laura is a beautiful actress, but still have to prove herself in a big movie. I think she can be a cool Bulma.
Yamcha - Daniel Wu Yamcha is a dead horse in DBZ. He's there just to be killed and fail. Maybe a talented actor like Wu, can make him more relevant.
Chichi - Cecilia Cheung Goku's wife, and Gohan's overprotective mother. All she wants is that her son doesn't become a delinquent. Cecilia is a good actress. She could be a very nice Chichi.
Master Roshi - Ken Watanabe The Martial arts master, who was the strongest man in the world a long time ago and now is a wise and pervert old man. Serious some times, funny in anothers, the right guy to play this role is Watanabe. The guy is an awesome actor and everybody knows that, so case closed.
Nappa - Nathan Jones Nappa is the rude, cocky, brainless guy. Muscles over brain. Nathan have the build to play this role.
Raditz - Takeshi Kaneshiro The first challenge in DBZ, Raditz is Goku's older brother who came to earth looking for his weak young brother. He is cruel, rude, and is kind like the exposition character here. Kaneshiro is a veteran in the film industry, especially in China. He's also a singer and voice actor, being the voice for Samanosuke Akechi from Onimusha and Tarzan's Voice for Cantonese, Mandarin and Japanese versions of the disney animation.
Kami Sama - Joe Morton The green alien turned into earth's god, and creator of the Dragon Balls. Kami is a very complex character, with a rich backstory. I don't know why, but i think that Joe have the look to play Kami Sama.
King Kai - James Earl Jones The ruler of the North Galaxy, and Goku's final martial arts teacher, and later mentor. He possesses great wisdom and knowledge about the universe and is a specialist in universal telepathic links. James Earl Jones, the voice of Vader, can be awesome as King Kai, especially in moments where the telepathic communication is used.