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. appearing in The Incredible Hulk, and continued with recurring appearances by Samuel L. Jackson and Tom Hiddleston as Loki.
Villains have also made surprise returns, such as Samuel Sterns in Captain America: Brave New World, 17 years after his last appearance.
The Marvel Cinematic Universe has always been built around its interconnectivity.
Author's summary: Ultron's return may ruin the Avengers villain.