I wouldn't be surprised if Yelena becomes the new Black Widow and takes her late sister's superhero name in her honor, letting Romanoffs continue the legacy.
She already is the new Black Widow. It's not a superhero name -- it's the name of every assassin who came out of the Red Room, from Dottie in Agent Carter to Natasha to Yelena and all the others we saw in the movie.
And she's not a Romanoff (or more properly Romanova), she's a Belova. They aren't biological sisters -- it was a childhood cover identity that they chose to embrace later in life.