While it was thought Robert Downey Jr. would be done with Marvel following The Avengers: Endgame, now it is being reported Tony Stark will be back for Black Widow.
In Deadline's report about Marvel raking in the Saturn Awards, it's noted (bold my own):
The finale installment of the Marvel big-screen saga was named best comic-to-film release. Robert Downey Jr.’s swan song performance as Tony Stark, aka Iron Man, in Endgame earned him the best actor in a film award. (Downey will be seen in the role of Stark one more time, however, in the Marvel prequel Black Widow in May 2020.)
Since Black Widow is a prequel set before the death of Tony Stark in The Avengers: Endgame, obviously Tony is alive and the use Robert Downey Jr. makes sense, as well as "cents" as the inclusion of RDJ will obviously be pretty huge and a big draw for a movie that doesn't have a big-name cast, except for Scarlett Johansson.
Black Widow has been said to be taking place between the events of Captain America: Civil War and Avengers: Infinity War.
Marvel also released concept art at Comic-Con showing off new costumes (see below).
Rachel Weisz, who plays Melina Vostokoff, also recently said that her character has been cycled through the Black Widow program five times, having started out as a child and that Melina is a highly accomplished spy and assassin, and that there are many Black Widows in the film.
Black Widow kicks off Marvel's Phase 4 and has a May 1, 2020 release date directed by Cate Shortland also starring Florence Pugh as Yelena Belova, David Harbour as Alexei Shostakov / Red Guardian, with Taskmaster yet to be cast.