right now I'm working on my corpse factory au for them, but if I had to make something completely original I think I'd revisit an old concept related to brain parasites. Whether it's an au where Childe's abyss brain damage manifests as something like that, or Scara getting some brain condition that makes him hallucinate Childe.
I like things where Childe seems to be possessed by something as well. Like, him and Scara having a normal relationship, but whatever is possessing Childe views him completely differently. Failed exorcisms, conflicts where the entity is able to mimic Childe to briefly trick Scara, stuff like that.