Re: Love Interest for Captain America II
Natasha is one of SHIELD's top agents. Sharon doesn't necessarily have to be... she can be the wide-eyed, youthful agent. Or not. Either way, it doesn't matter. These movies do largely stand on their own, and they can be taken either in context or watched all on their lonesome. The reality is that there are 1000s of SHIELD agents, and the Avengers don't call all of them in. No need for the fanwanky explanations, just tell your story with your characters and let the audience assume the others are otherwise occupied.
Also there is absolutely no sexual tension between Steve and Natasha in Avengers. So, no, she can't do everything Sharon can. The personality of the characters matters about a billion times more than whether or not their jobs have superficial similarities.