YESSS, I tried to leave this open ended, but there were a few directions I was kind of thinking someone could take this in for MAXIMUM HILARITY--one was jealousy, like you said (poor poor Jack, experiencing An Emotion), and another was..."Zhao Zi has apparently adopted this kid, and now I'm expected care about TWO people? Unrealistic. Blocked." (Also just a more general "the kid investigates their relationship Very Seriously only to find out that literally everyone else already knew about it" kind of deal, lol.) Really any variation on this could be GREAT imo, and I'm also really excited about the idea of someone with good comedic writing to take it for a spin!

An angsty version could be really great too, though, and I can see a great fic in which Jack and Zhao Zi have to confront some aspect of their relationship that becomes highlighted by the tension introduced by this new person (either because jealousy or him asking questions or w/e). Really there's no bad direction to take this prompt in!!