What would be cool and interesting crossovers with Discworld & Non-Discworld characters? Personally, I think the concept of Hogwarts students(from Harry Potter) going to the Unseen University would be rather funny.
The Librarian when they try to go in the Restricted Section:
Ron: Why is there a monkey in here?
Hermione: The librarian being an orangutan was never mentioned in Hogwarts: A History
Harry: I think it (sorry, he (sorry, esteemed orangutan)) wants us to leave.
Fred and George: An orangutan in a library? Why didn’t WE think of that?
Hagrid: Isn’t he so sweet?
Luna: Did you know, Librarianus Orangutanus is actually rather common. In fact, this Librarian is actually Voldemort, who accidentally got transfigured irreversibly when he visited the Zoo whilst babysitting the England Quidditch team captain’s nephew.