I think they'd just match each other's freak really well. Though Mu would either hate or really love that Shion is mostly motivated by ruining just one guy's life. She'd actively want to make her worse.

I think he might be taking a break from theater for a bit. While he's at the Akademiya it feels like he's not trying to attract too much attention to himself, or he seems a bit focused on studying/writing his essays. It's so funny he's doing historical stuff and his source is just himself.

I feel like he'd enjoy watching more theatrical stuff, which is funny because it puts him in a similar situation to what Furina is in right now.

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.