People are saying this Dr. Doom is actually Wanda, because the mask matches the one from the Latverian witch from San Diego. But this mask looks different from both the witch's and the Dr. Doom we saw in the trailer so why is that?
But why does Doom need Scarlet Witch? He needs her to build him a new reality, much like she created a city full of brainwashed people except Doom will scale it up to create a brainwashed world: Battleworld.
Doctor Doom will take one of his harbingers to Hell with Steve Rogers. Why Steve? Because Steve knows about Cinthia, which is why he'll stay by Doom's side right up until the face reveal..
There's a reason Mephisto is in the MCU now. It will connect to Cinthia Von Doom's soul, and that will tie into Ghost Rider's entry into Secret Wars or Doomsday will at least tease him.
If Dr Doom isn't at the Fantastic Four tower for manipulation, and is instead just there to take Franklin to 616 in order to revive Scarlet Witch, then that's a different scenario. But I still believe Dr Doom or a future version of Strange took Scarlet from there.
Why Agatha? Because she is directly connected to Cinthia Von Doom, who is one of the first witches in the world and Agatha All Along makes that clear. Also, her losing her son, and Doctor Doom losing his mother to Mephisto and Death, ties everything together perfectly.
Also, they're changing the color of her cloak. While that doesn't disprove the Cinthia theory since she did wear these colors in the comics it does suggest that Doomsday will involve more than one witch.
That doesn't mean we won't see that happen, nor does it mean I'm against the idea of him losing to Reed. But that on its own just doesn't have a strong narrative foundation.
I talked about this in one of my theories way before. Doctor Doom's mother arc is important for Doomsday and Secret Wars because it would be a better plot for introducing Doom than the defeating Reed Richards arc.