The Marvel Cinematic Universe is known for its interconnectivity, with major characters making unexpected appearances in various movies.
Examples include Robert Downey Jr.'s surprise appearance in The Incredible Hulk and Samuel L. Jackson's recurring roles.
Villains also make return appearances, such as Samuel Sterns in Captain America: Brave New World 17 years after his last appearance, and Tom Hiddleston's Loki.
The studio made this clear when Robert Downey Jr. appeared unannounced in The Incredible Hulk.
Author summary: Ultron return causes concern.