One of the main issues with creating a dragonball based rpg is the source material itself. Roleplaying games are a very social event, where many players are involved. Traditionally the storyteller/ gm creates a scenario and the players use their creativity to solve the problem. Dragon ball is not team focused. Each time a big bad arrives, the heroes usually get destroyed and one character stands up to the threat alone. Most battles in fact are one on one. In order to look at this game, you have to consider not doing dragon ball as it is alone.
What you need to do is take the flavor of dragon ball. Maybe you want large epic battles with heroes and villains who can shatter mountains and fly at super sonic speeds. An example of how heroes this powerful work together is actually DC super heroes. Epic 1 on 1 battles are just not very fun when it comes to role playing so the entire structure must be considered. For a dbz style game, there has to be teams of fighters battling. That is why I mentioned DC. Many times teams of villains fight teams of heroes. Something like that has to be present for a social event like an rpg. I am currently working on a way you can have a single super boss without outclassing one or more players to the point where they cant hit anything.
One issue I have with the source is that humans are essentially to be discarded. A lot of how dragon ball z works is that an alien saves the day. Not very many examples exist of human martial artists besting the villian are available. The issue with this is that the game sets you up to fail. In order to become better than a human (for example a sayian) you have to roll a 2 on 2d6. What this leads to is 90% of the group gets to play humans, who will eventually be worked out of the story. My suggestion (and something I wish to consider for my rewrite) is to find a way to keep all players involved. If the game has mixed humans and aliens, find a way to keep the heroes in the game. Toriyama himself said that the only reason he came up with the Kami and Piccalo fuzion was because he wanted android 17 and Piccalo to be on even grounds. Secret items and training techniques might save the day. For example, sacred holy water in dragon ball, perhaps a new strain of senzu bean could be developed to enhance benefits of natural trainning? Maybe you could avoid humans entirely and play a heroic Sayian campaign. That way, when someone becomes a super sayian, the other players would work even harder so they all can get to that level. Maybe you could play low level enough to avoid super sayains entirely.
These are some of my considerations while I am working on my rewrite. If anyone wants to drop a line and discuss, please let me know. If anyone has any good suggestions, I am also all ears.