Kang is out and they are apparently throwing out that storyline. The new title is: Avengers Doomsday.
The big bad is Doctor Doom and he is played by none other than Mr. Robert Downey Jr.
Link with a few details (this was all apparently just announced at San Diego Comic Con):
https://screenrant.com/avengers-5-movie-title-doomsday-doctor-doom/
I don’t get it. Are we supposed to ignore the fact that Doctor Doom looks exactly like the late Tony Stark?
Same actor. Different metal mark.
A clip from the reveal of RDJ:
I mean, if Ryan Reynolds can play two completely different characters named Deadpool and not even Wolverine noticed, then why not?
Have you ever seen Captain America and The Human Torch in the same room at the same time?
I think the idea is that the Multiverse allows for it. Either we’ll never see his face and he’ll always be masked, or he is the Tony Stark of a different timeline/universe.
Or Buzz Lightyear.
It’s fairly to imagine a Tony Stark who goes ‘too far’ and ends up as the villain.
Well… 
IIRC from the comics, Doom’s real face is so horribly scarred that it wouldn’t look like Tony Stark anyway.
You can make the argument that it’s happened twice already in Age of Ultron and Civil War.
I’m picturing Spider-Man hearing his voice for the first time. “Mr. Stark?”
“…yes, that’s correct; I can’t explain right now, but you need to believe me when I say that everything depends on you bludgeoning Sam Wilson to death as soon as possible.”
There aren’t any other actors available?
What is this, Law and Order?
Doctor Doom is known for monologuing, so the role could be perfect for Sam Waterston.
As others have opined, maybe it’s a multiverse thing? Honestly, once you introduce the multiverse to a fictional universe, nothing much matters anymore. You’ve jumped the shark. The exception would be if the whole point is to explore existentialist themes. So, fair enough for Rick and Morty or Everything, Everywhere, All at Once, but pretty much a death knell for Marvel IMHO.
That said, since they have introduced the multiverse to Marvel (and foolishly gone on to produce additional works in the same continuity), what the heck? Go ahead and make RDJ the villain and the hero. Heck, make him every character! There are literally an infinite number of RDJs and Tony Starks available in the MCU.
I don’t have any problem with this casting choice at all. If a Cinematic Universe can have different actors play the same role, it can have the same actor play different roles.
It’s called “acting” for a reason.
Jumped the Stark.