I feel semi-certain we are all reading this (as well we should) but I have to throw some love out there for Highway of Diamonds (http://archiveofourown.org/works/759841/chapters/1421451) by alphaflyer Seriously, if you're not reading it, you're missing a very smart, very interesting story. Summary from the horse's mouth: The road to danger is often lit by shining stars. Clint and Natasha are sent to investigate a secretive cult, and discover things about each other in the process. Mission fic; pre-movie.

I'm also really digging I've Learned How to Paint My Face (http://archiveofourown.org/works/575186/chapters/1031131) by ashen_keyfor it's spot on depiction of terrifying, deadly tween Natasha and the interesting situation everyone is put in when she's unexpectedly de-aged.

I know there are so many more but I'll have to think on it.