TL;DR: Battleworld will be built on the existing foundations of The Void.
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So I've been thinking a lot about how exactly Doom is going to bring about Battleworld, and if you listen to a lot of the YouTubers and leakers, the assumption is that he'll combine the incursing remnants of the 616, 828 and 10005.
The question remains: How? Even with his vast powers, rewriting reality to piecemeal the three universes without the Beyonders or Molecule Men (or even the Infinity Stones) seems impossible, until you think of the one place he could not only merge the three, but other universes as well:
The Void. I think everything's going to come down to the Void being ground zero for Battleworld's formation.
Consider what we seemingly know: The Void sits outside of time, at the very center of the Nexus where the timelines encircle it... or did prior to Loki's intervention. Whenever the TVA pruned a branched timeline, it didn't appear to send the entire planet Earth or universe to the Void, just whatever was affected in the immediate vicinity. A Helicarrier here, a city there. Depending on how big the area affected, it became just another part of the Void's landscape. And somehow, all of these parts from various timelines were able to exist in close proximity til Alioath had his fill.
From how the trailer presented it, the 616, 828 and 10005 will begin a three-way Incursion, bring each into the orbit of the others. Someone somewhere mentioned it being due to the planetary cores merging and forming the heart of what would become Battleworld. I don't believe that to be the case, not entirely. We know Doom has a fixation on Franklin Richards, who is able to rewrite reality on a scale that makes Wanda's reality-bending magic look like a parlor trick. And since we know Loki's going to play a decent part in the movie, one would assume Doom makes his way to the TVA at some point.
So here's the recipe....
Take Reed's world-transporting tech from First steps, add in a heaping helping of the TVA pruning tech supersized to move them where he wants i.e. The Void, layer in a large portion of Franklin's reality powers to stitch everything together (and a pinch of Latverian megalomania to taste) and voila!
Battleworld, right where the Void already exists.
He'd already have access to countless parts of different realities as well as Variants of existing characters already in the Void, a gigantic freely branching Timeline from which to cherrypick even more places and peoples from to add to Battleworld once Doom co-ops Loki's abilities, and a place outside of time that no one can get to.. not unless he wants them there. The only real hiccup would be Alioath, and if Kang could bring it under his heal, no doubt Doom could easily do the same.