My daughter's bulletproof subgenre is "they're forced to live together." College roommates AU, roomies at the academy (whatever the academy is for them), only one hotel room available, etc. This is complicated by her being aromantic and not interested in ship fic of any sort - she wants buddies, or enemies maybe becoming buddies. (OTOH, this is a narrative kink she loves so much that she'll put up with some shippy fic to read it, as long as it's heavy on the dialogue and snark and doesn't just become romantic as soon as they realize they're interested.)

I went looking for those stories for her and was much annoyed to find out that there's no tag, not even a set of tags, that works to find them.

She also adores, "characters from canon visit our 'real' universe."

For myself, I love fics that have a twist that puts all of canon in a new perspective. This includes "he was a vampire all along!" but also "they were secretly married" and "actually, the protagonists were the villains." But that may be too broad a genre. (Too bad; we could call it "Everything You Know About Canon Is Wrong.")

Examples:
Nocturne by Tira Nog (Harry Potter, HP/SS)
Through the Glass by dentalfloss (MCU)
A Way So Familiar by Teland (DCU Comicsverse)