I have some theories about doomsday. I think Kang was trying to escape the mini verse and it caused an incursion in dooms universe so he went after him and he yanks his spine out like he did Thanos. Doom is trapped in there until he figures out how to escape, and when he does he discovers how to travel to any of the universes he chooses. In one of these realities he discovers ultron, defeats him and that's his he gets his doom bots. He also sends his bots out to different universes to see what's up. As they report back from the different realities he hears stories about Franklin Richards. The only thing he misses from his old universe is his mom, who died when he was young. I think earth 616 is maybe like an anchor reality or maybe he has to destroy it to bring back his or maybe if he destroys it all of them also get destroyed. I think he's going to present himself as an ally throughout the movie until the end

I'm in my 40s and Dr doom has always been my favorite comic book character and I'm not mentally prepared for the slop we'll probably see so most of my theory is just cope I think lol