One thing about the DBZ characters, I find them slightly owerated: Although they can blow up whole planets etc, it mostly involves one or more of them diing. Come to think of it, every few episodes or so one of them dies.
Another thing is that even as strong as they are, they get beaten up by new enemies regulary, having to train or learn some new flashy beam attack to be able to beat them. I think it wouldnt be that hard to include them and still keep it balanced.