Well how do i start this?..
I'll just start with the main point, i want us to find Clint Barton apart from S.H.I.E.L.D in Avengers : AOU
Like Erik Selvig in the Thor : The Dark World, How Loki screwed with his brain making him sort of leave S.H.I.E.L.D and be a little.. crazy, i feel the same thing should go for Hawkeye, but less craziness all-together.
Kind of like Bourne Legacy in a way it would be cool to find him sort of hiding out, Brazil.. Africa? somewhere bizar and far away, Scruffy beard? a little bit like Bruce Banner in The Inrecdible Hulk.
After the events of the Avengers, he leaves S.H.I.E.L.D (explaing no appearance in any phase 2 films) getting his shit together, this would also give him a bigger role (kind of) and maybe make his story more interesting, and cooler, (being alone apart from S.H.I.E.L.D could build up his character)
Now there's one option, regardless i want that too happen (although i didn't break-down into that section or go into detail that's kind of a rough idea of where he could be.)
Another option, depending on how Captain America : The Winter Soldier ends maybe Black Widow starts working with Hydra like the comics? With a reason behind it? and isn't actually completely bad.. Maybe she leaves S.H.I.E.L.D all together and is doing something mysterious, and LIKE the comics (or TV show) Hawkeye, who would leave S.H.I.E.L.D due to finding Black Widow, goes after her leading to places like Africa or somewhere?
ALL THAT ASIDE -
Or both? maybe he's getting his shit together hiding out while trying to find out where the hell Black Widow is? maybe it could lead to Quicksilver, Baron Strucker?
Regardless,
This would give Clint Barton a cooler, (like said earlier) and more significant and remembered role.. Flash-backs of things he's done, Loki in his head and when it all comes down to it, The Avengers need him they find him in Brazil? (anywhere) Africa 'Baron Strucker or Black Panther?? ;)' and maybe by that time he figures he needs to grow stronger then the demon inside his head. I sort of made it sound like 'eventually he gets his shit together' No, no, no.. it could happen early on, not towards the end of the film, not like he thinks - "Hey, they really need me I'll go help"
That could, and should happen but not towards the end where everything is just nuts.. but maybe middle of the film or even before that?
Him being gone (and possibly after Black Widow 'if that even were to happen just an example') Could really tie in with the story that's involves Baron Von Strucker, Quicksilver and Scarlett Witch.
*** Sorry for being all over the place, and i hope you get what i mean, comment where you think his place in the Film should be and if you agree ***