As of writing this answer there has not been anything official in regards to confirming future Iron Man franchise movies being greenlight. However, Marvel Studios head Kevin Feige has touched on the subject of new actors / recasting roles, etc.
We have a new Spider-Man right now who was in our “Civil War” film and is in “Spider-Man: Homecoming,” and the audience has embraced it. And you can look to Sherlock Holmes, James Bond, and Batman as characters that last longer than any one actor playing them. There’s a precedent for it in other franchises that suggests it’s possible. But right now I don’t want to think about it and don’t need to think about it."
Variety via Comicbook.com
"Well, if I believe Tony Stark to be James Bond, then there would be an Iron Man 4, 5, 6, 7, 8, 9...". But without Robert Downey Jr? "That's the question. That's a question we won't have to face until after Avengers 3, so that's at least five or six years away".
Den of Geek.com