A Different Idea for Hellsing.
Basically, a crossover including small details from Bloodborne. So Queen Analise from Bloodborne uses her Hunters to collect Blood Dregs to create a child of blood, say she'd been saving those dregs for centuries, and over the centuries Cainhurst was lost to the world around it and she had that child. That Vileblood Prince would be Menma, and my thinking on how he would operate, ability wise, would be similar to Vladimir from League of Legends, a Hemomance who also indulges greatly in blood.
Chuck that kid in with a Hunter, Maria/Plain Doll as caretaker and the sacking of Cainhurst, ending up with a Prince and his servant chucked out into the real world and exposed to capture by Hellsing. Goes from there basically.
A Bit of Chaotic Fun
Ok so pretty much a fragment of the Emperor's Soul from Warhammer 40k finds a new home inside Menma's head. First Psyker on Elna is born and the consequences that come with it.
A Potentially Good Idea
Fire Emblem Awakening story, Menma as a Dark Wyvern Lord, Basically a Wyvern Lord + Dark Mage, with a halberd, and the super spooky magic. The Shepherds come across him during their escape from Plegia, just after Emmeryn mercs herself, and manage to escape due in large part to him and his big fuckoff mount covering their retreat. He ends up tagging along with them.
Obviously it'll have more substance to it than that but you get the drift.
Pairing will either be MenmaXTharja or MenmaXLucina. One because I fucking love goth chicks and two because Lucina is a bit of a cutey.
And because I always pair the others off whenever I play anyway.