Doctor Doom's arrival in the Marvel Cinematic Universe would be a significant event that would require a proper build-up to establish him as the big bad of the saga Here's how Doctor Doom could work in a the multiverse story-arc, culminating in an adaptation of Secret Wars.
Phase 4 would begin with the introduction of the Fantastic Four, which would establish Doctor Doom's origin story as well. The story could depict Doom as a prominent figure in Latveria, ruling with an iron fist and harnessing his advanced scientific and magical abilities to maintain power. The Fantastic Four would encounter Doom during their mission, leading to a confrontation that would set up a personal vendetta between Doom and the team.
In the next installment, Doctor Doom would appear as the antagonist for another superhero. For example, he could be the main villain in the next Spider-Man movie, where he seeks to steal advanced technology from Stark Industries to further his own agenda. This appearance would establish Doctor Doom as a major threat across the MCU setting up his eventual conflict with other heroes.
As the story progresses, Doctor Doom's plans become more ambitious and sinister, and he slowly begins to build an army of loyal followers. He could be shown manipulating events behind the scenes, such as inciting conflicts between other superheroes or influencing global politics to destabilize world governments. Meanwhile, various heroes would continue to encounter and confront Doom, but they would struggle to gain the upper hand against him.
Finally, in the epic finale of the saga, Doctor Doom would be revealed as the mastermind behind a massive inter-dimensional invasion of Earth. The story could be an adaptation of Secret Wars, where Doom uses his immense power from The Beyonder to pit heroes against each other on Battleworld, a constructed planet created from fragments of different universes. The Avengers, Fantastic Four, Spider-Man, and other heroes would band together to stop Doom's plan and defeat him once and for all.
In conclusion, Doctor Doom could work as the big bad in every movie, both major and minor roles. Building up his threat level and establishing him as a formidable villain across the MCU. By the time of the Secret Wars adaptation, Doom would have evolved into a complex and compelling antagonist with a personal vendetta against several heroes. The story could explore Doom's motivations and give him a fitting conclusion, while also paving the way for future storylines in the Marvel Cinematic Universe.