Man, I’ve seen a lot of Impmon fic over the years, and it’s wild how many writers just slot him into the classic 'grumpy x sunshine' dynamic. Like, he’s obviously the grump, paired with someone endlessly cheerful—Renamon seems to be the most popular, but I’ve seen it with Terriermon or even Henry. It’s an easy way to get that friction-turning-to-affection beat. The other big one is the redemption arc fics, where he’s being slowly brought back from the dark side post-'Tamers' by someone’s patient love, which usually goes hand-in-hand with hurt/comfort scenes.
Less common but way more interesting are the 'unlikely partners in crime' AUs. I read this one where Impmon and Leomon were con artists running a digital-world scam, and the weird buddy-cop chemistry was hilarious. Those feel truer to his chaotic gremlin energy than just making him a softie for someone. There’s also a weirdly specific niche of 'Impmon adopts a human' fics, which always end up being surprisingly sweet.
Honestly, the most overused trope for him is the 'tsundere' act—all that 'I don’t care about you, baka' stuff that melts after three chapters. I wish more people played with his pride and his deep-seated fear of being weak again, instead of just making him a pint-sized grump with a secret heart of gold. Still, you can usually find some decent takes on AO3 if you filter carefully.