I hate to say it, but the fanon that drives me the most nuts is Clint living in the air ducts. I'm not opposed to the use of them in missions (either of the serious or prank variety), but he must have better things to do besides crawl around all day...

For tropes, nothing gets me like a good fake relationship or undercover-as-married story. Oh, and secret marriages, too :)

I probably have a million headcanons, some I've written about and some I haven't (yet):

1. Natasha secretly loving to be in the kitchen. I usually say baking because I'm a big believer in baking as a method of stress relief and also because it's something she'd never have learned in the Red Room, so it's a little thing she does for herself and herself alone.
2. I like to think of Clint as well-read from picking up books along the way while being in the circus; like, maybe he's not great at fractions, but he's read Plato, or something along those lines. I also tend to think of him knowing a lot about astronomy and astrology from that time in his life.
3. I firmly believe, for no real reason, that both Clint and Nat love karaoke. I also believe that Clint's pretty musical and plays the guitar, and that whenever he sings in Natasha's presence, he changes any names in the song to hers.