it's fascinating how much variety there is under the same ship. A major pull is the doppelgänger or 'mirror match' dynamic. Writers love exploring the tension between the 'original' and the copy—is it rivalry, resentment, or a strange, twisted understanding only they can share? You get a lot of fics about identity crises, with one trying to prove he's more 'real' than the other, which inevitably spirals into intense emotional confrontations.
Another huge theme is the forced partnership or 'enemies to co-workers to something more.' They're stuck on the same heist team or assigned to the same impossible mission, and the constant bickering slowly morphs into reluctant respect, then vulnerability. It's a slow-burn goldmine. I've also noticed a surprising number of supernatural or sci-fi AUs—vampire verses, cyberpunk settings—where their rivalry is grafted onto a power struggle in a wholly different world. It lets authors play with the core dynamic without being constrained by canon.
Less common but really memorable are the quieter, introspective pieces. One that stuck with me was a fic where they meet by chance in a laundromat at 3 AM, both running from different things, and just talk. No grand heists, just two tired people who look identical recognizing something broken in each other. That kind of character study is where the ship really shines for me.