Well, I've only written in two licensed universes to date, Trek and Marvel, and only twice in the latter. Although I have two ongoing original-fiction universes and a smattering of standalone original stories.
Meanwhile, I'm determined to leave no universe untouched by my grubby little hands.

Without naming names, I was actually approached once about finishing a book by a dead author, but the money being offered wasn't worth the amount of work required, and it wasn't a series I was dying to write in, so I passed on the project.