Depending on her reception in Avengers I think Black Widow is a strong possibility.
If not then I could see Marvel wanting to stay close to their Avengers formula but still pursuing a female led film and go with Spider-Woman. They get the best of bot worlds in Avengers tie-in AND a character loosely tied to a property they can't as of yet touch in Spiderman.
If not then I'll take something like Black Panther, Ka-Zar, Man-Thing or Moon Knight!!!