Sure, there have been films like Captain America: The First Avenger, and Ant-Man, which took the action back in time, but there has yet to be a Marvel film that fleshes out the backstory of an established character in true prequel fashion.
Although it’s not the most likely project, a Doctor Strange prequel centering on the life of The Ancient One would certainly represent a step in a new direction for Marvel.
For the sake of mixing things up, it definitely seems like an interesting potential move for the studio.
Despite the fact that the Marvel Cinematic Universe is incredibly complex and interwoven, it has not really delved into the realm of prequels yet.