Just to let you know Goku does leave the main group at the end of DBZ to train with Uub(The reincarnation of Kid Buu)for three years but he never actually leaves Earth so your idea fits fine with the official story. Also if you were talking about GT(which I haven't seen)then that doesn't really matter because GT isn't canon. I do have a question to help establish a baseline so our characters aren't op. What kind of range do you want the characters to be in. Do you want us to be below where a super Saiyan would be? Dragon Ball and DBZ have many milestones for character power that you could use as a baseline.