Robert Downey Jr. will reportedly return as Tony Stark / Iron Man in Black Widow, the next Marvel Cinematic Universe film centred on Scarlett Johansson's title character, also known as Natasha Romanoff. [Spoilers ahead for the MCU.] If you're wondering how that's possible considering what happened to Iron Man in Avengers: Endgame, that's because the standalone Black Widow movie is a prequel — naturally, considering Romanoff shared the same fate as Stark. The extent of Downey Jr.'s role is unclear. It's possible that he may not film any new scenes for Black Widow and will instead be shown through archival / previously unused footage, as has been the case for other Marvel stars and films in the past.
That's likelier given how Downey Jr.'s possible Iron Man return has been reported. In its coverage of the 2019 Saturn Awards over the weekend, which noted Downey Jr.'s win as best actor, Deadline reporter Geoff Boucher remarked in a parenthetical addition: “Downey will be seen in the role of Stark one more time, however, in the Marvel prequel Black Widow in May 2020.” Considering how big the news is on its own, it's curious that it was buried in a story that doesn't even have Downey Jr. in the headline, let alone mention his possible MCU return. That heavily suggests Downey Jr. has likely not been to the Black Widow set, and that his role will likely be limited to a cameo appearance.
Black Widow is set after the events of 2016's Captain America: Civil War, per earlier reports, so Marvel could go back to its archive around that point to bring Stark back. The prequel is also expected to give us a peek at Romanoff's origins, so it could go further back. It's possible that Downey Jr.'s return will be restricted to a flashback sequence in Black Widow, considering the shared history between Stark and Romanoff. Johansson made her MCU debut in 2010's Iron Man 2 as an undercover S.H.I.E.L.D. spy posing as a Stark Industries employee who becomes Stark's personal assistant on Nick Fury's orders.
Since then, Johansson has appeared in six MCU films, including 2012's The Avengers, 2014's Captain America: The Winter Soldier, 2015's Avengers: Age of Ultron, 2018's Avengers: Infinity War, and the aforementioned Endgame, and Civil War. Some are terming Black Widow as Johansson's swansong, though it could easily produce sequels of its own if Marvel is pleased with its commercial prospects.
Black Widow will release May 1, 2020 in India and elsewhere.