Zero_619
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Don't overthink bro just do it.
Can't imagine the harem being small with the fanfic being multiverse level so here are few Rias, Akeno, Kuroka and Yasaka from DxD and Wonder Woman, Power Girl, Talia al Ghul, Zatanna and Raven from DC
How about starting a family after the 4th great ninja war. Unless the MC wants to gain immortality or try and find ways for dimension travel to go to other worlds.
*cough* Indra *cough*