It's interesting because a lot of the fics I see treat their relationship as this ghost haunting Mafuyu, you know? The dynamic rarely sticks to just flashbacks. Writers love exploring the 'what if'—what if Yuki hadn't died, what if Mafuyu had noticed something sooner. A ton of AUs pop up where Yuki is alive but they're estranged, or they're stuck in some supernatural loop together. The evolution often hinges on Mafuyu's guilt; he either learns to live with it and let Yuki's memory become something gentler, or he gets consumed by it in darker fics. I've read a few where Mafuyu literally talks to a ghost-Yuki, and the evolution is just the ghost fading as Mafuyu heals. It's less about romance evolving and more about grief transforming.
Some fandom corners get really meta with it, treating their past relationship as this perfect, untouchable benchmark that Uenoyama can never meet, which creates its own angsty evolution in present-day fics. The relationship doesn't evolve between the two of them so much as Mafuyu's internal narrative about it does, and fanfiction is obsessed with picking that apart from every possible angle.