I've read a lot of Geno/Reaper stuff, and the emotional conflict usually feels pretty one-note to me? It's almost always a revenge cycle thing—Reaper killed Geno in 'Undertale' so now Geno is fueled by vengeance, Reaper is consumed by guilt or doubles down on being a jerk, they fight, there's angst, maybe a begrudging team-up or a weirdly romantic tension if it's a ship fic. It gets repetitive. The better writers skip the obvious 'you killed me' drama and dig into what that death actually meant. Like, Geno isn't just a victim; he's a timeline resetting, determined survivor who's seen countless endings. That paranoia versus Reaper's single-minded, fatalistic purpose could be way more interesting than just 'I hate you for killing me.' I saw one fic that framed it as a mutual haunting—Geno, a ghost of a possibility, haunting Reaper's present, and Reaper, as this embodiment of finality, haunting Geno's every attempt to change things. That stuck with me more than any fight scene.
A lot of the conflict also gets externalized through Sans, which can be a crutch. Like, 'Sans is worried' or 'Sans tries to mediate' becomes the B-plot instead of letting the two of them just... exist in their messed-up dynamic. I prefer when the story makes their conflict so internal it warps reality around them—glitch effects, resets bleeding into each other, that sort of thing. Shows the emotional toll without having them yell about it. Honestly, the fics that handle it worst are the ones that try to make Reaper secretly soft too fast. The emotional payoff is hollow if you don't earn that shift from antagonism to something else, you know?