Honestly, the development arc for Mafuyu and Yuki in fanfiction often feels more compelling to me than a lot of official side material. A ton of authors pick up right after that initial, tense encounter in 'Given' and just run with it, using the fanon space to explore the awkward, silent healing they'd both need. You see a lot of fics where Mafuyu starts tentatively sharing music he's working on, not Uenoyama's songs, but his own rough melodies, and Yuki's ghost becomes less of a haunting presence and more of a bittersweet memory they can finally talk about.
It's rarely a straightforward romance. The progression usually hinges on Mafuyu learning to separate his grief from his identity, and Yuki learning to see Mafuyu as his own person, not just his dead brother's boyfriend. The best ones have them bonding over small, mundane things—coffee runs after practice, complaining about band logistics—building a new history that isn't overshadowed. I've seen a few daring takes where they don't end up together romantically at all, framing their evolution as a profound, platonic understanding that's somehow even more intimate, which I find really refreshing.
That said, some writers rush the reconciliation, and it feels unearned. The ones that nail it let the silence between them speak as loudly as the dialogue.