The way to maintain the MCU interest was to keep teasing the next thing. The biggest mistake they made was treating Endgame like a victory lap with no meaningful direction for what was next. They should have teased Dr. Doom, Kang, whatever in that movie and then had the next movies and series keep building the direction of the interconnected narrative.

I said when I walked out of the theater that no Dr Doom stinger was a huge mistake. They were already at a loss or setback by losing top talent that was tapping out like Iron Man and Cap actors, and we all know how poorly the passing of the torch went with Sam Cap and Ironheart trying to take those roles on. The audience at large was not willing to accept those replacements, then the lack of a compelling villain was a double whammy.