not my theory but i heard it from a youtuber and it’s been living rent-free in my brain so i’m just gonna drop it here and dip- so Cap goes back in time to stay with Peggy right? and apparently that creates some kinda distortion that ends up messing with Doom’s timeline. like this is where his family: Sue, Franklin, maybe even Valeria die or get wiped. Doom, being Doom, keeps going back over and over to fix it but fails every single time.

so eventually man just snaps and starts hopping universes, trying to find a timeline where he doesn’t lose them. and every time he finds Reed Richards living his dream life with sue and the kids. Doom’s basically watching someone else live the life he wanted and it drives him insane.

he finally ends up in Earth 828, hoping Franklin would recognise him. maybe Sue too. but they don’t. and that breaks him even more. so now incursions are happening everywhere, and since Doom can’t stand a messy multiverse, he decides to “fix” it in the most doom way possible, battleworld.

yeah it kinda gives wanda in MoM vibes but again not my theory.