
The Fantastic Four: First Steps' mid-credits scene finally revealed the first look at Doctor Doom, and he has one target in sight: Franklin Richards. Victor Von Doom (portrayed by the returning Robert Downey Jr.) is set to make his presence felt when he goes up against the Earth's Mightiest Heroes, the Fantastic Four, and the X-Men in Avengers: Doomsday. While not much is known about his grand plan, many have theorized that it has something to do with the Multiverse and the incursions that are already happening.
Meanwhile, the new Fantastic Four movie introduced fans to Reed Richards and Sue Storm's son, Franklin. The newborn baby is deemed important and powerful, which is why Galactus (the Devourer of Worlds) wants him for himself. This isn't surprising because Franklin is a key player in the grand scheme of things in the Multiverse in Marvel Comics. For those unaware, Franklin Richards is an Omega-level mutant who has the power to alter reality and create pocket universes.
After managing to defeat Galactus by transporting him to a different part of their universe (with some much-needed assistance from the Silver Surfer), the mid-credits scene jumped four years into the future, where a four-year-old Franklin Richards is seen cuddling up with his mother, Sue Storm, during story time.
After Sue sets off to find another book for them to read, she uses her force field abilities to sense Franklin. She later finds Franklin seemingly talking to someone, and it's none other than an unmasked Doctor Doom.
Doctor Doom's arrival in the MCU is chilling, raising questions about why he chose to reveal himself like that to someone like Franklin Richards. Still, it makes sense because of Franklin's reality-altering abilities, and it could play a crucial role in his sinister plans for the Multiverse.
Here's Why Doctor Doom Wants Franklin Richards & His Power

In the mid-credits scene, Doctor Doom seems to be on the verge of kidnapping Franklin Richards, which could kickstart the events of Avengers: Doomsday.
Franklin's reality-altering powers, combined with the confirmation that he has the Power Cosmic within him, are the main reasons why Doom wants the powerful child to begin with, and it has something to do with his nefarious goals to save the Multiverse in his own twisted way.
Doom could manipulate Franklin and use him to achieve dominion over the Multiverse. Given that Franklin is still an innocent child, Doom has all the means to easily control him and encourage him to do his bidding.
In 2015's Secret Wars, Franklin used his god-like abilities to help restore the Multiverse with the help of his father, Reed, and Molecule Man. Given Franklin's role in that story, one would assume that Doom wants Franklin to use as insurance and keep Franklin locked up somewhere to prevent him from stopping Doom from achieving his goals.
After The Fantastic Four: First Steps' ending established that Franklin can revive the dead (after restoring his mother to life in the aftermath of the fight against Galactus), it is also possible that Doom wants Franklin to revive someone important whom he can use in his plan for the Multiverse, and this mystery character could potentially be Wanda Maximoff (Elizabeth Olsen).
Although Wanda's fate after Doctor Strange in the Multiverse of Madness is still up for debate, Doctor Doom could not know whether Wanda is dead or alive, so he decides to keep Franklin if he ever needs to revive the Scarlet Witch.
But how does Doom even become aware of the actions of Earth-616 Wanda or Franklin's reality-altering powers? The four-year time jump could explain that Doom has been studying the Multiverse, with Galactus' arrival in Earth-828 serving as the launchpad for his curiosity about why this Devourer of Worlds wants Reed and Sue's baby in the first place. Given his sheer genius intellect, he could've also become aware of the incursions that are happening.
In Marvel's Secret Wars, Molecule Man was created by the Beyonders as a "living bomb" to destroy realities. The death of Molecule Man would accelerate the destruction of the universe. The character also served as the "living battery" of Doom's Battleworld - a planet composed of fragments of destroyed realities where the villain serves as king and ruler.
In the MCU, the Scarlet Witch has been established as a destroyer of worlds, and what better way to put it than by taking the place of Molecule Man in 2015's Secret Wars. Doom could seek Wanda because he could be planning to use her as the power source for his planned Battleworld. If Wanda is truly dead, then Franklin could revive her, essentially accelerating Doom's plan.

By trying to find Wanda, Doctor Doom and Franklin Richards could end up on Earth-616, and this could explain why the Fantastic Four of Earth-828 can be seen heading into the mainline MCU (as shown in Thunderbolts* post-credits scene).
By piecing it together, the Fantastic Four's arrival on Earth-616 would alert the Avengers and the New Avengers to the incoming Multiversal crisis and its incursions, giving them a reason to team up and try to stop Doctor Doom once and for all.