A recent teaser for 'Avengers: Doomsday' has sparked discussion about a major Marvel fan theory, with some outlets suggesting that Robert Downey Jr. may be playing Doctor Doom. The possibility of Downey Jr.'s return and the introduction of X-Men characters to the MCU have also been mentioned in relation to the film. However, details about the movie, including its cast and plot, remain unclear, with various outlets reporting on potential spoilers and rumors.