I can't help wondering how dark Joss would have gone had it been one of his tv shows rather than a big budget Hollywood film. He's gone to some pretty dark places with all of them, and I think if he'd had the same freedom with Avengers Clint may have tried to kill Natasha as per Loki's command. She would still kick his butt/save him, though, because Natasha is just that awesome and she ain't going to be shoved in no fridge.

I think the potential would definitely be there to go very dark. And a part of me kind of wants this in fic form just to see how it would all play out (deftly handed, of course, and with respect). But it's an incredibly compelling, fascinating, horrible premise.