If you want to do a messageboard RPG let it be freeform but with certain finite rules, from what I have observed (and experienced) is that genuine role-playing in a DBZ rpg is usually crushed by mindless godmoding. Without rules you will find that the roleplayers' characters will be continually smacked around by godmoders until they get tired and start godmoding back. So while characters should be able to do all the DBZ stuff (transformations, fusions, training, tournaments, saving the planet, hunting for dragonballs, comdeic interludes, etc.) you should avoid this sort of thing:
Player (Playing Goku): Vegeta and I transform into Mystic SSJ4 with kaiyoken x100 and fusion, then we find all the dragonballs and wish for immortality and then invincibility. We blow up the planet cuz we r L33+, lol! lol! lol!
"Yahaha...It's burning...Melting...Our only chance...To see hope die--Ahhh! My brain--Numb with beauty! Ha ha ha ha!" - Mr. Buick, Battle Angel Alita: Fallen Angel