It looks like Marvel Studios are slowly getting things into place as they have added two big names to join the Black Widow movie.
The first to join was revealed by The Hollywood Reporter as Stranger Things star David Harbour and this was followed by Variety breaking the story that Rachel Weisz is being looked at for a major role.

Following her recent Oscar nomination and BAFTA win for The Favourite, Weisz next move could be to join the MCU standalone film as the US trade states the deal hasn’t been completed, but talks are going well and heading in the right direction.
It’s unknown who the British actress will play, but rumour is Weisz could be the secondary villain.
As for Hellboy star Harbour, THR is also being kept quiet on his possible role with it being speculated online that he could be playing a role leaning towards the comic relief side or maybe as the primary villain.

For this moment Marvel is keeping a tight ship as Avengers: Endgame is upon us but we should here info soon after as production is set to start in London in June.
The same goes for the plot but is basic stuff is the story will be internationally set and centring on Scarlett Johansson’s Natasha Romanoff. The question is if it’s a prequel focusing on the spy/assassin’s origins being brought and trained by the KGB before becoming an agent of SHIELD and Avenger. Or if the film is set after Endgame.
Harbour and Weisz will be joining rising British star Florence Pugh and will be directed by Australian filmmaker Cate Shortland with the script written by Jac Schaeffer.