It's been 100% confirmed that Kang the Conqueror is the main villain of this set of phases.
The next Avengers movie is even going to be called Dynasty of Kang or something like that.
Yes, that is what I mean: three years (with a release date of 2025) is a wildly compressed time frame to try and do another event movie on the same level as Ininity/Endgame in terms of setup, especially when this is all seemingly starting in one of their smaller properties (Ant-Man makes money but it isn't drawing the most eyes).