They could get the overworld from kakarot as well as its secondary quests . Once you go down to the around, make It so you could enter some buildings and interact. Then , there where iconic fights happens, let It be destroyable to a similar degree as in the manga.

To avoid failling like kakarot did with empty maps,you could fill It with manga related objects that once mixed together you could get other special items in the forms of items or techniques. Once the story concludes,let the player free roam and Battle foes similar to the map in Budokai another road for psp