For an AU dream share, I was thinking either Homura could be a student adopted by Caster, maybe helping to serve as an extra source of energy willingly, or maybe Caster could be a mage in Homura's world and Homura is trying to recruit her due to her own credo. And to mostly avoid any killing if she sees Caster is a bit similar to her; she got betrayed by someone she loved, though he was gleefully evil and she broke every bone in his body. Could also work for the Fate/ AU like her not keeping the secrets guarded enough and disowned from the family.