The Marvel Cinematic Universe is known for its interconnectivity, with major characters making unexpected appearances in various movies.
This trend was set early on, with Robert Downey Jr.'s unannounced appearance in The Incredible Hulk, and continued with recurring appearances by Samuel L. Jackson and Tom Hiddleston's Loki.
Villains have also made surprise returns, such as Samuel Sterns in Captain America: Brave New World, 17 years after his last appearance.
The studio made this clear when Robert Downey Jr. appeared unannounced in The Incredible Hulk
Author's summary: Marvel's interconnectivity may ruin Ultron's return.