Hold on, I have to laugh a little because I think we've all seen the exact same arc in a lot of these. The sheer amount of 'gamer boy who’s actually a cinnamon roll with severe social anxiety meets deep-voiced, intimidating-looking guy who’s actually the gentlest, most protective bean' is honestly a genre unto itself. It’s that classic 'opposites attract' but with a heavy dose of 'everyone’s perception is wrong.'
A huge theme is the hidden softness, especially around Corpse. Writers love exploring the contrast between his voice and presumed edgy persona with moments of quiet vulnerability—like him having panic attacks that Sykkuno helps him through, or being secretly a huge romantic who writes poetry. Sykkuno is often the 'light' that draws him out of a dark place, which ties into a lot of hurt/comfort. You also get a ton of 'online friends to lovers,' where the fic is built around them realizing their feelings during long, late-night Discord calls, with all the awkward, sweet realizations that come with that.
And I’d be remiss not to mention the sheer volume of AUs where they’re rivals in something—streamers, e-sports, even coffee shop baristas—with that intense competitive tension that inevitably melts into something else. The fandom really latches onto their dynamic from the Among Us era, that specific blend of Sykkuno’s chaotic innocence and Corpse’s focused intensity, and just expands it into every scenario imaginable.