The Marvel Cinematic Universe (MCU) is known for its interconnectivity, with characters often appearing in unexpected movies or making surprise comebacks.
Notable examples include Robert Downey Jr.'s unannounced appearance in The Incredible Hulk and Samuel L. Jackson's recurring roles throughout the franchise.
Villains also make unexpected returns, such as Samuel Sterns in Captain America: Brave New World, 17 years after his last appearance, and Tom Hiddleston's Loki, who has appeared multiple times throughout the franchise.
The Marvel Cinematic Universe has always been built around its interconnectivity.
Author's summary: MCU's interconnectivity raises concerns about Ultron's return.