Tonguez
A suffusion of yellow
I think it's going to be more like the lead-up to the modern Secret Wars event, where the Heroes try to stop the incursions from happening but Doom realizes that that's futile, but he can steal the power of the Beyonders and use it to put together a world from the remnants of the multiverse, with himself as God-King.
And I mean, that's fair enough, but I think Doom shouldn't be coming out of nowhere. He deserves better. Thanos was teased for a long time before appearing as the main villain in Infinity War, even if he actually showed up in only 2-3 movies. And it makes more sense for someone like Thanos to come out of nowhere, while Doom works best if he has history (particularly with That Fool Richards; and of course with Luke Cage).
It would be a fun angle if Doom comes across as a heroic saviour in Doomsday, despite Reed warning everyone not to trust him and the Avengers unsure - because he looks like Tony!
Do the whole first movie with Doom as hero of Latveria and leader of the worlds heroes - Doom has his Day!
Then Secret Wars is part 2 where Doom outmanouvers the Beyonder and becomes the tyrant that all the world rallys against