The time has come for me to finally answer this! My answer is, without a doubt, Meteor Impact!

Though it may seem like a confusing answer at first considering Midori barely makes an appearance, I assure you it's the story that paves the way for midochia's future. Meteor Impact solidified that Chiaki's story can only become complete with Midori, and Midori's story wouldn't exist without Chiaki.

The nuance for us as readers—to know why Midori has made it his purpose to peel away every layer of Chiaki's mask—wouldn't exist if we weren't aware of the significance of these walls in the first place. The reason we know what Chiaki's glasses represent, and the implications of Midori's infatuation towards Chiaki wearing his old glasses; it is only because of Meteor Impact, that we can feel the depth of their relationship.

It ties the context of every midochia story together. In Study Heroes when Midori first learns Chiaki is more than he presents himself to be. In Sweet Halloween when Midori got glimpses of a vulnerable Chiaki, panicked at the idea of showing his weak side. In Comet Show when Midori wouldn't allow Chiaki's years of hard work to wash away and let him leave the stage he knew he adored.

During Climax, when Midori showed Chiaki what it means to be a hero—with a tokusatsu quote no less—it is because of Meteor Impact we know what that meant for Chiaki, the very same Chiaki who could only shyly encourage himself with tokusatsu phrases all alone in the past. For all the time Chiaki had desperately clung to hope, when he could only endure and wish for a brighter future; that very moment of Midori thanking Chiaki for all he has ever done for him is what Chiaki was waiting for. Though Chiaki never dared to wish for a reward, it was all of Chiaki's rewards culminated.

Though Midori doesn't know the exact details of Chiaki's past, he has long ago realized the reason Chiaki clung to him was to give Midori what Chiaki wished he had in the past. And in the future to come, when Midori gets through to Chiaki that he not only became a hero because of Chiaki, but for him—to be the hero Chiaki never had—it will be because of Meteor Impact we'll know what that truly means.

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!!

Good questions earn descriptive answers, so that's what I'm gonna give you! First of all, Childe deserves every type of body jewellery there is but my favourite might be tongue piercing! Childe's outfit is the perfect blend of work-appropriate (grey-scale uniform) and playful (tummy peak and red scarf), and that's why a tongue piercing suits him perfectly - a simple metallic ball that you wouldn't notice unless you're looking for it. A navel piercing in a similar style fits nicely too, only noticeable when his jacket moves just enough to expose it.

Of course, I'm a sucker for marking Childe's body so I would make him get even more piercings! The order of which got the piercings would probably be something like ear>navel>tongue>nipple>dick. His first ear piercing he made himself with a needle at the age of 14. From time to time he adds another ear piercing until he stops at 11 total (hehe), 6 on his left ear and 5 on his right. Except for the ear piercings I don't imagine him wearing any other very visible piercings like lip or nose ones.

Oh, I just realized the question was about body jewellery but I went wild with just piercings. Other body jewellery at his ever-growing jewellery case would be a variety of thigh straps, chokers, ankle bracelets and rings. Though I say all that I don't imagine him wearing much jewellery unless someone else dresses him up, at least in the canon universe he seems very practical and not too bothered by appearances. Luckily we know of someone who would make sure to spoil him at every opportunity. (ꈍᴗꈍ)