I do have a considerable amount of trust in Joss Whedon. (Huge Buffy fan, here. :)) Heck, he's the reason I finally watched The Avengers in the first place.

but I have hope that we can at least persuade Marvel to give us Natasha's film.

Honestly, I would bet money that that's not gonna happen. Look at the lineup. There's no room for a BW movie any more. Hey, I would LOVE to be proven wrong. Please, PTB, prove me wrong!