Well similar to making your own monsters, say for example:
You take the strongest Player Character and use their stats to make a Creep(Creature) Make up a small story for him, have a quest Npc start you on the quest and have a huge reward for those who can kill him, or Create an npc out of thin air. Give it insane stats and make a quest for it. That in my opinion will keep the high lvs entertained, and give something for the lwr lvs to try and shoot for. Not to mention Players would be dying to try and get npc's created that look like them. I mean I know it's a Dbz game and it's nice to stick to the cannon story, but in my opinion it would be cool to have something the Cannon doesn't. (Besides the mass amount of Player Characters that were not in the cannon at all.)
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Play your way to better health.
Having a pwr lv of over 9k helps too >.>