There was a lot of speculation early on that EVC might be playing Sin, but it seems to have died down.
However, I have always maintained that there is something mighty squirrelly with the fact that Agent 13 has NEVER been referred to as Sharon Carter in a single official press release from either Marvel or Disney. Every single time we have seen EVC's character referred to as SC it has been fan speculation.
I, for one, would love to see a female villain in a Marvel film. Might mess with Cap's head, which is always a good thing. The more angst the better, I say.