Ok but ENOUGH about Kuroo whispering Kenma’s name while fucking Tsukishima (i’m in love with that headcanon tbh) BUT what about:
Kurotsuki fuck-buddies. No feelings. But Tsukki starts noticing that Kuroo is  someone you can actually talk to, he’s surprisingly mature and kind. And Kuroo is starting to realize that Tsukki’s reactions to everything makes him smile like an idiot. They are falling for each other, of course they are. But Kuroo thinks Tsukki loves Yamaguchi and Tsukki thinks Kuroo loves Kenma. They are both too vain and arrogant to ask, it’ll hurt their prides if the other figures about their true feelings. So they keep it cool. Seeing each other, having dinner together, fucking like crazy, cuddling… And that’s when it hits; “What am i even doing, he’ll never love me, he loves someone else, I don’t want it to be over, I am so stupid, this is stupid”. Whenever Tsukki sees Kuroo laughing while hugging Kenma his heart breaks a little. Whenever Kuroo sees Tsukki so talkative and smiling to Yamaguchi his heart breaks a little. They can only fuck, feelings are not allowed. Dates are darker now and sex is not that good when the only thing you can think about is the other thinking about someone else. Eventually they give up on each other. (Kuroo cries a lot srry)