I believe this is already available, when you access a file there is a Fork and Edit button that's creates the fork, let you edit the file inside the browser, let you create a commit and a Pull request.
I believe currently the fork is not delete after the PR was merged so this need to be done manually.

--- Edit

Ok I read this too quickly :), so what is missing is the possibility to delete the fork after the PR was merged.

But if the fork exists and is old, you end up editing an old file. Or if the file doesn't exist in your old fork, you end up on a home page of the original repo.

@pingou @cverna what if we have a old fork and we update the old fork and make that PR, I am thinking about deleting the fork is that really necessary?

I's not about deleting a fork.

From my experience, I'm almost never able to edit anything on thr web, because my existing fork is completely out of sync. An exceptions are repos I've never contributed to, therefore I have no fork.

What if the "edit" button created a branch that would be in sync with whatever I want to edit?