One hc that captivates me is modern au Xiao having religious trauma.

His previous experience with a god could be reframed in a modern au as him being part of an abusive cult. And he's saved by Zhongli (whom is a different person from Rex lapis obvs in this scenario) who introduces him to whatever religion Rex Lapis has going on in this scenario and Xiao getting like... Really into it.

Like, to the point of basically replacing the cult that he grew up under with Rex Lapis kind of thing. And he becomes, like, more devout than Zhongli himself in that.

I can see him taking literally any commandment or religious teaching to it's logical extreme. Like "Abstain from excessive pleasure" becomes to him "abstain from basically any pleasure at all". And he does this precisely because he's grown up to associate religions with extremes and that if a God commands something he should do it to the fullest.