I agree, I don't really have a static headcanon either, although I do have a main one.

I also like the idea of it taking some time, and wrote it that way in my own 'first meeting' fic. I actually haven't written much established relationship fic at all for them yet.

Also, I'm currently toying with the idea that in my MCU fic 'verse, Clint dated, married, and divorced Bobbi all between bringing Natasha in and the events of the movie. I think I'm going to have to fic that. :)