The concept of Kaeya getting his slyness and mischievious streak from Crepus. Like, he was already playing mind games but Crepus did not help curb this habit.
Also crack headcanons again. This is not how I think it went down but it would be so funny if it did:
The idea of Crepus finding Kaeya at the winery and brought him back to the city with no explanation except "What do you mean 'What is that'? You've seen a child before. This is my youngest. I've always had two sons?"
Varka: "Are you just....fucking with the entire city?"
Crepus: "Yeah. This is really funny"
Varka: "I...How is it working?"
Crepus: "Mondstadt's winds carry my words in mysterious ways- Oh I hear someone coming"
*hides in the corner of the bar as the door slams open*
Cyrus: "SINCE WHEN DID CREPUS HAVE A SECOND SON?"
Cyrus: "You know! Blue haired, darker skin, skinny, adorably small?! The whole town's been saying that the boy is Crepus's?! Since when??"
Varka: "How is this news to you. You've been his acquaintance for years. Kaeya's just a year and a half younger than Diluc, remember?"
Cyrus: "what are you TALKING about."
Varka: "It would sadden Crepus to hear that you've neglected his youngest, you know. He hates it when people play favourites when it comes to his kids. Better plan something grand for the little guy as reimbursement,"
Cyrus: "WHEN DID HE GAIN A SECOND?"
Varka: "You don't remember the birth? We were all so overjoyed,"
Cyrus: "Am I drunk?? What is happening,"