It hasn't been completely confirmed yet, but we know that there's a very, very good chance that Marvel's Black Widow is one of their Phase 4 projects and that Scarlett Johansson will return for a prequel story dealing with how she comes to be aligned with SHIELD.
In the comics, that story is tied to the story of Bucky Barnes as The Winter Soldier, and it would make a lot of sense that the movie would introduce the same dynamic - particularly as Sebastian Stan has films left on his MCU contract (and probably WON'T take over as Captain America, no matter how much fans want it).
Stan seems enthusiastic about that possibility too, as he spoke to Bad Taste to say that he wants a return for “a full-on Winter Soldier" if Black Widow’ does end up being an origin story.
“Yeah, absolutely! Of course! I mean, anytime, I’m ready... They have a very nice history, in my opinion. It’s very intricate … The truth is, he actually taught her in the comic books. He was a teacher to her. It’s all set during a specific, interesting time. The Cold War. It’s all a very … spy, thriller-ish, noir-ish story. It would be very interesting to explore that on film. I would love if they do that. You know, I hope. Maybe they will.”
That would be a great approach, especially as it would give us more of Stan as The Winter Soldier - perhaps as the villain alongside the likes of Arnim Zola and some other Black Widow students as Natasha goes rogue.
In another universe, we would have got to see more of the link between the two characters in the MCU, as there was originally a scene in The Winter Soldier that explicitly mentioned Natasha's past more, which would have teased her origin story and tied it to Bucky's past even subliminally:
For now, we'll just have to wait and see whether Marvel makes the right decision.