Oh gosh, where do I begin! I'll name just a couple ones I can think of on the spot~

Immovable rock x unrelenting tide!
I like a feisty and bratty Childe who has to expend his energy before he tires, softens and becomes putty in Zhongli's arms. I like a patient and tolerant Zhongli who enjoys being the witness and cause to these changes in Childe. And taking care of him too, of course!

Touch-starved and emotionally constipated.
I like Zhongli who hasn't indulged in carnal or domestic pleasures before (it never occurred to him as something he craved when he was an Archon) and Childe who will recoil at any touch that isn't from his family or from combat. On top of that, Zhongli who has yet to understand the intensity and complexity of mortal emotions, as well as Childe with self-destructive tendencies and a habit to disregard his own wants and needs.
I want them to fall into a pattern of casual touches leading into more, only to realize "oh, this is what I've been missing from my life".

Then there's like... arranged marriage. I just love it, no reason. I'm always looking for more!!