I see this pairing pop up a lot in post-canon scenarios, more so than any other theme. Everyone wants to know what happens after that final scene under the tree, right? The fics that grab me aren't the ones that just jump straight into a fluffy established relationship, though. I'm drawn to the ones that wrestle with the actual, messy work of building a life together when both of them are still learning how to be open. How does Yuu navigate her own uncertainties when she's not just following Touko's lead anymore? How does Touko handle not being 'perfected' and just being a regular, sometimes insecure girlfriend? The best stories dig into those quiet negotiations, the small moments where they have to unlearn old habits. Like, a fic where Yuu hesitates to voice a criticism because she's scared of 'ruining' Touko, and Touko has to consciously stop herself from performing gratitude. That stuff feels real.
Another huge theme, and honestly my personal favorite, is role reversal or 'what if' AUs. Not the coffee shop or high school ones—those feel too generic for them. I mean scenarios that flip a core dynamic from canon. There's a fantastic one I read where Touko is the underclassman who admires a confident, seemingly-put-together upperclassman Yuu. Watching Yuu, from Touko's perspective, grapple with the same imposter syndrome and performativity is a mind-bend. It highlights how much of their original dynamic was about perspective and timing, not just personality. Those AUs are popular because they reaffirm why the ship works by testing its foundations in a different sandbox.
Finally, you can't talk about themes without mentioning the hurt/comfort tag. It's massive for them, but it's very specific. It's rarely major, dramatic whump. It's emotional hurt/comfort, almost exclusively. Touko having a panic attack about her acting career and Yuu knowing exactly how to ground her without words. Yuu feeling overwhelmed by the pressure to always be the 'steady' one and Touko, clumsily but earnestly, trying to reciprocate the support. The comfort is quiet, often a shared silence, a held hand, a forehead touch. That's the core of their appeal, I think—the communication that happens in the gaps between words, and fanfiction thrives on exploring those gaps.