Since Doctor Doom debuted in 1962's Fantastic Four #5, Doctor Doom has grown far too big to fit into any one Marvel superhero's rogues gallery. While the character started out as the arch-nemesis of the Fantastic Four's Reed Richards, the supervillain has gone on to antagonize the rest of the Marvel Universe at one time or another. And when Doom makes his inevitable entry into the Marvel Cinematic Universe, that's why he doesn't have to debut in the inevitable Fantastic Four film.
The problem with introducing Dr. Doom in a Fantastic Four movie is that it already feels played out. In all live-action adaptations of Marvel's First Family, the armor-clad Latverian despot served as the primary antagonist. When Spider-Man made his jump to the MCU Marvel made an effort to distinguish his first film from his two prior cinematic iterations, and it wouldn't be too unlikely for Marvel to approach the FF's nemesis with the same strategy.
Given Marvel's currently revealed Phase 4 strategy, there are several places where Doom could realistically debut. With his multi-faceted nature an appearance in each movie could emphasize a different aspect of his character, and while any of the upcoming properties would be perfect for him there are two front runners that play up very different sides of the character.
DOCTOR STRANGE IN THE MULTIVERSE OF MADNESS
Doctor Strange in the Multiverse of Madness promises to have a world-changing magic and dimension-hopping setting, which could set the stage to play up the magical aspects of Doom's character. While in the comics Doom's scientific genius is surpassed only by Richards, his main rival for magical know-how is Doctor Strange. If Doctor Strange 2 wants to play up a massive threat to the MCU on par with Thanos, then Doctor Doom would be the best way to go.
Such a rival could echo Roger Stern and Mike Mignola's classic Triumph and Torment story, where the two doctors became unlikely allies against an ever greater foe, but the multiverse of possibilities the film's title promises means anything is on the table. One of the more sympathetic motivations for Doom provided over the years is his desire to save his mother's soul from hell, and if the Sorcerer Supreme's duty is to preserve the status quo that could easily put him in contention with the main doctor.
BLACK PANTHER 2
Doctor Doom's genius is not limited to the spheres of magic and science, either. He is also a political genius, ruling over his homeland Latveria with an iron-gloved fist that firmly maintains order. His status as a geo-political threat would make him a natural fit for Black Panther 2, which takes place after the Wakandan Kingdom involved itself and its advanced technologies in global politics.
Vying against Black Panther would be a refreshing way to introduce Doctor Doom to the MCU, and plays against many of the ideologies and themes the hero supports nicely. While T'Challa's motivation for publicizing Wakanda's advanced technology was toward the end of helping to save the world, Doom hoards his mind-boggling innovations to himself for his own gain. Pitting the Wakandan King against the Latverian Dictator could bring out the best of both characters, and they already fought each other in the 2010 crossover Doomwar, which could inspire their cinematic battles.
While those two films seem like the main contenders, there's truly no limit to where Doctor Doom could fit in Phase 4. Black Widow could explore Doom as a political threat in a more intimate cloak-and-dagger way than two warring kingdoms, while WandaVision could place Doom in more of a puppeteer role as he manipulates the Scarlet Witch's power and madness toward his own ends. There's still no telling what shape Eternals will take, but the cosmic race of space gods probably hold some power Doom covets.
After all, Doom's lust for power is a frequent motivation for his character that often threatens the entirety of the universe around him. If Thor: Love and Thunder or Guardians of the Galaxy Vol. 3 were to similarly involve power of cosmic proportions it would be a safe bet that Doom wanted it for himself. The MCU's proclivity to draw inspiration from the comics is well established, and God Emperor Doom was one of the most massive moments Marvel Comics has ever seen.
Given all of these possibilities, it almost seems limited to think of Dr. Doom purely as a Fantastic Four villain who needs the super-family to set the stage for his debut. Especially given the post-credits shake-ups the MCU loves to throw, such as the "Avengers Initiative" mention that set up the whole connected universe, it's anyone's guess where we may see the famed green cloak and metal mask first.