Carmilla and Zestial? The dynamic practically writes itself because of that whole ancient, aristocratic vampire thing they've got going. A lot of fics lean into the 'immortal ennui' trope, where they're both so old and tired of everything, but find a spark of something new in each other. You see a ton of slow-burn political marriage AU scenarios—they unite their respective territories or broods for stability, and of course, feelings develop against their better judgment.
There's also the 'enemies to reluctant allies to lovers' pipeline, which fits perfectly with the Overlord hierarchy. Maybe they're forced to work together against a common threat like Heaven or a rival Overlord, and all that tense, strategic interaction morphs into something else. The 'hurt/comfort' variant is huge too; one gets injured, and the other, against all their cold exterior, is the only one who can or will provide proper care, revealing a hidden vulnerability.
I've noticed a niche but recurring theme of 'shared memories of the Old World.' Fics will have them reminiscing about pre-modern times, comparing experiences in different courts or wars, which creates this deep, melancholic intimacy that no one else in Hell can really understand. It's less about flashy romance and more about two ancient beings finally finding someone who remembers the same lost centuries.