If you want to know my approach to this, it's to lift ideas from other games, books, movies, tv, comics, etc., and just adapt them to my own story.
Like I like Lady Rowena's approach of having every NPC in her FMs be related to every other NPC in some way, either they were rivals, or best friends, or secret lovers, or had a bad history, and there are readables all around that fill out all the different relationships which are fun for the player to find. So I try to do something like that in my stuff.
The best kind of smaller scale "events" for FMs are in the Thief 2 FMs by Eshaktaar, Lady Rowena, Lord Alan, Gaetane, Saturnine, Purah ... ones where every room or scene also has a little event or story by itself. TDM FMs have done this too, but I guess I'm a sucker for the classics. But don't discount the value of lifting ideas from movies and tv shows. Keep a little notepad around and when you see something write it down.