I am guessing this is going to happen:
They’re going to base this off the Jonathan Hickman Secret Wars run where there were incursions. Dr. Doom’s own universe, with his mother dying, was destroyed because of this. He discovers Captain America was responsible for this by living in another timeline. (I believe this was gonna be Kang’s plotline before they abandoned him. That was Kang’s story in Avengers: Earth’s Mightiest Heroes.) He is looking for a way to control the incursions. The X-Men’s universe gets destroyed and they come into the main MCU timeline as invaders, as they tracked Captain America to being responsible. In secret Dr. Doom tried to convince the X-Men that the Avengers were responsible for losing their universe and paints a target on Steve Roger’s back for them.
In the meantime the FF figure out what Doom’s plan is: to try and use the chaos of the universes colliding to take control of the universe himself, to bring back his mother. I imagine they’re gonna come up with some strange device to help them counteract Doom’s plan and save everyone. I imagine it’s gonna be them create a pocket universe.
Doom then manages to try and infiltrate the TVA and take control of the timelines, to which he intercepts alternate universe Loki. Defeats Loki. Doom manages to unlock unlimited power over the universe, to which Thor alone tries to confront and stop him. Doom defeats Thor, stopping his axe like in the trailer. Then, Doom accelerates the colliding of the universes. They all collide under Doom’s command, destroying everyone and bringing the end of the world, to which Doom then takes unlimited power as essentially a god. He thinks he has won. He starts to try and create a universe in his image.
However, thanks to the FF, they managed to create the pocket dimension and saved everyone through it. But they are trapped lest they come into Doom’s own universe of his making. End of the first film.
For Avengers: Secret Wars they have to finally stop Doom. I imagine they’re gonna adapt the 80s Secret Wars comic this way. They reintroduce Spider-Man and the Hulk and Dr. Strange here. They come into Doom’s new universe. Dr. Strange realizes what happens and tries to escape with the other heroes, discovers the FF’s pocket dimension. What ensues is that Doom follows them with his unlimited cosmic power and forces them into another pocket dimension, that being the Battleworld.
The X-Men are still hostile and want the Avengers and Steve Rogers dead, but the FF and the remaining heroes help and try to discover how to access Dr. Doom. Yadayada, heroes fight in the Battleworld, Doom probably brings new villains into the fold. Spider-Man gains the symbiote here. Maybe introduce some other Marvel heroes we haven’t seen yet like Spider-Woman?
Eventually leads up to a climatic battle with Dr. Doom and the FF where Doom is defeated. Franklin Richards becomes the key to all of this, with his universe manipulating power. He creates a new universe without Dr. Doom’s interference, and restores the homes of those who suffered incursions. MCU Reboot happens. The end.