"We'll continue to introduce new heroes, as we've done over the past few years," he said. "I just can't talk about which new heroes yet."
"It's impossible to predict what's going to happen," added Downey jnr.
The MCU has been slow to diversify; it took 18 films for a superhero of colour to lead their own feature with 2018's Black Panther, and 21 to have a woman at the helm with Captain Marvel.
But the future of this colossal cinematic experiment looks likely to make up for lost years, with their long-overdue first gay character expected in the upcoming The Eternals, their first Asian superhero expected in Shang-Chi, and more female-driven stories expected such as a solo Black Widow film.
"We're always striving to support our female heroines from the previous movies as well as this one, so we'll continue to explore that," said Endgame executive producer Trinh Tran.
"Looking at the [Endgame] poster... I see our very strong female characters standing next to the men, and I'm so proud that they're up there represented in that way."
Brie Larson said she recognised the importance of representation when Captain Marvel was released in March this year.
"Carol taught me so much," she said. "I trained for nine months before I started filming the movie. It changed my brain and changed the way I carried myself, and it strengthened my voice, so it was a great honour to be able to share that character with the world with the hope that others could get some of that through the screen.
"She is a symbol of the importance of representation, the importance of the female story and the female journey, but she's also for everyone. It's about equality at the end of the day."
Downey jnr said after trying to "make space" for himself with Iron Man, he's now thankful to be witnessing the changing dynamics of the MCU - particularly after the glass ceiling-smashing success of Larson's Captain Marvel.
"I'm glad I get to be part of witnessing this cultural moment that's coming up," he said. "I have a lot of gratitude, and just to be next to the lady of the hour, who's broken through this double-pane glass window and re-established what this genre is supposed to be, it's very gratifying."
And as Endgame brings a sense of finality to the last decade of the MCU, director Joe Russo said he's grateful to the fans who have followed every step of the way.
"This is the culmination of 10 years of a narrative experiment," he says. "If you've been following along for the last decade, and you've been putting your heart and your passion and your emotions into these characters, this is a very important movie for you.
"This is the ending, this is the finale, this will put to bed the story of the original six Avengers, so we have all worked exceedingly hard and put our hearts and souls into this film. We're all very proud of it, so we hope you like it."
