One idea could be that a new powerful character arrives in both the Naruto and Dragonball Z universes. This character has a unique charm that attracts the female characters from both worlds. For example, in Naruto, Sakura, Hinata and Ino might be intrigued by this new person's skills and different personality. In Dragonball Z, Bulma, Chi - Chi and Android 18 could also be drawn to them. Another plot could involve a time - travel scenario where a character from one universe travels to the other and creates a harem there, bringing together different fighting styles and relationships.
Well, perhaps there's a magic or some sort of ancient power that fuses the Naruto and Dragonball Z worlds. A young male character discovers this power and in the process of exploring it, he meets all the female characters from both series. He accidentally unlocks some hidden potential within them which makes them feel indebted and attracted to him. So, gradually a harem forms around him. He then has to balance their different personalities, like Naruto's friends who are more ninja - like and Dragonball Z's females who are used to a different kind of action - filled life.