Im more interested in a prequel than a sequel.
Hakan + Eli backstory, the first murders..
Eli in a different time with a different helper..
The man in the wig's + human Eli story..
Etc
Or expanding the universe of the novel itself, f.e. short stories about Oskar and Eli's "adventures" that were omited, or just mentioned in the novel, or just somwthing new,like O&E roaming around the building complex interacting with bizarre neighbors, more in the thriller aspects of the story, centred in the building, like "The Tenant" or Rosemary's baby-esque, or rewriting the story using a new character living in Blackeberg, or an existing one (like the lady with cancer).
An interesting crossover I did thought once was with another vampires, like O&E being adopt by the couple of "Only Lovers left Alive". Or the little couple traveling with the "Near Dark" gang.