For the love of books: Clint and Natasha work for a library. The kind of library where it's necessary to have the skills of a Master Assassin, and that's just dealing with the books never mind the visitors. Finding new volumes or retrieving over due books is challenge beyond belief.

Maybe like Terry Pratchett's L-Space libraries? Maybe like The Pagemaster? Like Dream's library in the Sandman? Maybe just a library that's dangerous because it's used by the X Men, the Fantastic Four, visiting aliens and Marvel or other verse characters?

Or just Clint and Natasha in a library, as part of a job or whatever. Just. Library love! :D