So, we think we figured out the plot of Avengers: Doomsday

Summary by Krater

This video breaks down seven post-credit scenes from the Marvel Cinematic Universe Multiverse Saga that set up the plot and characters of Avengers: Doomsday, including the introduction of Doctor Doom, splintered timelines, and the Fantastic Four.

From the video

Answers: What are the post-credit scenes in the MCU Multiverse Saga that set up Avengers: Doomsday?

• Avengers: Doomsday plot theories
• MCU Multiverse Saga post-credit scenes
• Doctor Doom origin and connection to Tony Stark
• Splintered timelines and the Sacred Timeline
• Fantastic Four and X-Men crossover in MCU

What it concludes

• Avengers: Doomsday will likely feature a cold open showing Doctor Doom's origin and connecting him to Tony Stark.
• The Sacred Timeline is a single universe containing parallel timelines that follow strict storylines and are pruned when they branch out.
• He Who Remains is the exiled Kang the Conqueror from the Quantum Realm who successfully isolated the Sacred Timeline.
• Doctor Doom in Avengers: Doomsday may be a variant of Tony Stark who was adopted by the Starks instead of dying in childhood.
• The Fantastic Four will cross over into the main MCU timeline from their home universe, potentially causing another incursion.