Yelena and Natasha going back to Russia is on my wish-list for a movie, but less "Natasha is framed for murder." In this case, one of the bad guys thinks Natasha killed her family... and she might not be wrong.
There are a lot of Russian superheroes and villains in Marvel Comics, obviously more common when the Cold War was going on. So, Yelena tells Natasha the Red Room has expanded into "creating their own superheroes." This includes Darkstar, Vanguard and Ursa Major. Maybe the Crimson Dynamo and Titanium Man, although they seem like Iron Man villains.
Given "Soviet Super Soldiers" is dated, let's go with Winter Guard.
Yelena wants Natasha's help to save these people from the Red Room, and teach them there is a difference between being a hero and a tool of the state. Natasha has to find her "inner Captain America" to give the right rousing speech to get the Winter Guard on her side.
But... Sergei Krylov of the Red Room has plans to sell the Winter Guard to Eric Williams of Williams Munitions. Krylov doesn't seem to care that Darkstar and Vanguard are his children. Eric Williams is better known as the Grim Reaper in the comics, but here he's just a businessman with an odd sense of humor. And an assistant/bodyguard named Nekra Sinclair.
The Grim Reaper wouldn't work as he was in the comics. He wouldn't hate the Avengers for "killing" his brother Simon, and wouldn't hate the Vision and Wonder Man for being "imperfect copies" of Simon. But I like the idea of Nekra fighting the Black Widows.
My casting call:
Eric Williams: Jason Bateman
Nekra Sinclair: Rita Ora
And Nekra got her strength and durability powers because she thinks Natasha Romanoff killed her family. Is she right? And Natasha doesn't get an "I was brainwashed" out.