I was thinking about this a bit and was thinking it should be Doom again, with a touch of the Doombots and Inhumans lore, but modified to fit the FF Movie universe.
So in the movie Doom uses reeds cosmic machine to change Grimm back into human form, and then Grimm uses it to revert back into the Thing form, so Reed's device is proven to work. This is a dangerous thing, a working device that can give anybody super-powers, and Victor being the brilliant and devious dude he is, and actually being around and utilizing the machine, will easily be able to recreate it.
So I was thinking Victor goes back to Latveria and reclaims his family's throne and he uses those resources to build his own cosmic machine for his own nefarious needs. His idea is to make his own army of super beings to do his will(replacing the idea of the Doombots) in his search to end the Fantastic Four and in the long run rule the world. One problem is he cannot, and will not use this device on his own people, since it would probably be considered an act of terror and he wants the adoration of his people, so he looks to another society to release this on. He looks no further than a neighbor of his, the small country of Attilan, which at this time is a smaller war-weary eastern bloc country with a low population ruled by a king named Blackagar Boltagon(later Black Bolt).
So Doom, without their consent, and unknowingly to them, cosmically 'nukes' the whole society blasting each citizen with the same cosmic rays that gave Doom himself his powers, masking it as a meteor crash or something akin to that. Reasons for this are as above, but also for an expirement as he is a scientist, and also for political gain, because as they all start to change panic and fear floods the small nation and Doom comes to their aid with financial and moral support, due to himself being afflicted, and wins their support for the time being.
The cosmic 'nuke' had an effect, but not equally on all people, turning some into powerful super-beings, some seemingly with no change at all, and some into underground dwelling sub-humans.
So Doom makes his case for opposition to the Fantastic Four, saying they will be a global police which is a threat to the free world, but Black Bolt and his royal family, including Medusa, Gorgon, Karnack and Triton, are unconvinced. Then Doom's savior comes in a seemingly un-modified citizen of Attilan, Black Bolts insane and jealous brother, Maximus. Maximus did actually get powers, the powers of mind control and posession, so he aids Doom in convincing his brother and the royal family into doing his bidding, and facing the Fantastic Four and then rallying his super-powered populace into taking over the world.
So after that Doom and the Inhumans would face the Fantastic Four and eventually through the battle Reed and crew would thwart the mental hold Maximus and Doom have over them to once again save the world, and free the Inhumans.
After this the Inhumans would retreat into seclusion and since their entire populace is modified they adopt the ritual of bathing their children in cosmic rays after puberty for all to be equal as this becomes their ritual from then on.
Sorry this took so long, just wanted to get out, what do you all think?
