Man, the Asriel/Frisk dynamic has been absolutely cooked to perfection in the fan spaces, and it's fascinating to see what people keep coming back to. The big one, obviously, is post-Pacifist 'Royalty' or 'Golden Ending' AUs where Asriel gets to keep his form and they both live in a peaceful, monster-integrated surface world. It's pure wish-fulfillment fluff most of the time, with Asriel adjusting to a new life and Frisk acting as his guide. You'll see tons of 'domestic' slice-of-life stuff: them going to school together, dealing with grocery shopping, that kind of thing. The angstier version of this is the 'Survivor's Guilt' trope, where Asriel is physically present but haunted by everything he did, and Frisk has to help him forgive himself—lots of tearful confessions in Flowey's old room.
Then there's the whole 'Time Loop Reset' category, which gets really creative. Sometimes Frisk retains memories across resets and tries to befriend Asriel in different timelines; sometimes it's Asriel who remembers and tries to avert his own fate. The 'Found Family' trope is huge too, with Toriel and Asgore becoming their awkward shared parental figures. A niche trope I've seen pop up more is 'Body Swap' or 'Soul Shenanigans,' where their souls get entangled after the True Lab events, leading to weird shared dreams or abilities. Honestly, half the fics feel like therapy sessions disguised as romance, but hey, after that game, they probably need it.