They didn't even attempt to explain Doom. He was in another dimension for over a year. No other apparent lifeforms, no apparent way for Doom to support his life.
So Doom must be some energy-eater who poops ozone. Or sucks on rocks for nutrients.
Plus zero explanation of what Doom did for a year, other than wait for them to come back. Then whats he want to do? Stay there... after destroying our dimension of course. Maybe grab a magazine to take back.
And how in the world does he just happen to be there when they show up? He's a scientist. Why is he not exploring once he realizes that he doesn't need sustenance or whatever other stuff that they just handwaved away?
During that year, Doom had other adventures involving Blaastar and Annihilus. He had learned the exact time they were returning by monitoring communications in our world during that period, so he timed being at the "pickup point" when that occurred. The reason he wanted to destroy our world and return to the negative zone is because of what he discovered during that year. This was all going to be explained in the sequel.
Got a source/link on that or is it just speculation?