I notice a lot of crossovers with 'D.Gray-man', actually. Both series have that Gothic, vaguely European aesthetic and an order of holy knights fighting monstrous beings, so blending the Kaname/Zero dynamic with Allen Walker and the Exorcists seems like a natural fit. The angst potential is huge—imagine Zero as a fallen Exorcist or Yuuki as an Innocence accommodator. The power systems mesh in interesting ways, too, with vampire abilities clashing with Innocence.
There’s also a surprising number with 'Vampire Princess Miyu'. The tone matches perfectly—that melancholic, doomed romance and the idea of vampires as tragic, beautiful guardians. I’ve read a few where Miyu encounters the Night Class, and the conversations are all about the burden of eternity. It’s less about action and more about that shared, lonely atmosphere.
And you can’t forget 'Hellsing'. It’s almost a meme at this point to drop Alucard into Cross Academy just to see the pure chaos. Those fics are usually crack-treated-seriously, exploring how the V.K. vampires' rigid, aristocratic society would implode facing an unstoppable force of nature like him. The contrast in vampire lore is the whole point.