The Marvel Cinematic Universe is known for its interconnectivity, with major characters making unexpected appearances in various movies.
This has been evident since Robert Downey Jr.'s unannounced appearance in The Incredible Hulk, and Samuel L. Jackson's recurring roles throughout the franchise.
Villains have also made surprise returns, such as Samuel Sterns in Captain America: Brave New World, 17 years after his last appearance, and Tom Hiddleston's Loki, who has popped up multiple times throughout the franchise.
The Marvel Cinematic Universe has always been built around its interconnectivity.
Author's summary: Ultron's return may ruin the Avengers villain.