Ryuko and Senketsu are such a unique ship because they're literally bound together, so a lot of fics explore that intimacy. There's a ton of body-sharing and mental connection tropes—like, scenes where Ryuko can feel Senketsu's emotions directly, or they have dreams that merge. It's less about romance in a conventional sense and more about this profound, symbiotic bond becoming something deeper. I've seen a few that play with the horror side of that, where the line between person and garment gets blurred in unsettling ways.
Another big one is post-canon 'what if Senketsu stayed' AUs. Writers get really creative with the mechanics—maybe he reforms from leftover fibers, or his consciousness lives on in Ryuko's scarf. Those stories often focus on domestic fluff, them just navigating daily life together, which is sweet considering the chaos they came from. You also get a fair number of fics that re-frame their entire relationship as explicitly romantic from the start, which can feel a bit forced to me, but the ones that let it develop slowly from that foundational trust are usually stronger.
Less common but memorable are role-reversal ideas, where Senketsu is the one piloting Ryuko in a sense, or fics that delve into his perspective during the moments he was dormant. The pairing naturally lends itself to themes of identity, sacrifice, and found family, so even the tropiest fics have a layer of weight to them.