Fics "in line with canon" or post/pre canon?

Looking for good fics that are either in line with canon, sequels or prequels. Basically by in line with canon I mean kind of like Levi's no regrets or Annie's lost girls. Stories that take place during the story but don't contradict anything, and can be read like side stories. Obviously prequels & sequels speaks for itself. Fine with OCs as well, just no one shots unless it's very long.

+1 if any of the warriors, Marleyans, Levi squad or lesser used characters are more prominent characters.