I'm pretty certain I'll have to abandon working with the fanon versions of Mari and Amy and just immediately export their backgrounds to new characters in my headworld, but what's crazy is I actually have decision paralysis because at least three headworld settings could take them in easily.

Kalhallaran was the first thought - Immune Cells already resemble The Disc in so many ways (giant sapient cells with insane interdimensionality and "antibodies") - but there's no incentive to 'experiment' on a human... but others would (like the Red Hearts guild, Ovo, By Dark, Brother Mend, etc.)...