That vibe has permeated all of Marvel’s Disney+ shows so far. The penultimate episode of WandaVision focused heavily on the character of Agatha Harkness - previously a comedic sidekick character, but actually a powerful and immortal witch who had been pulling the title character’s strings throughout the series.
The notion that a character might appear in one series and then leap to another or perhaps get their own solo book, enhanced that feeling that everything in Marvel was interconnected and part of some unseen master plan. These special guest star introductions also added to the sense that the Marvel Universe was a massive storytelling tapestry that was ever-evolving and expanding. It also kept things fresh in Marvel’s pre-existing books Spider-Man can only fight Doctor Octopus so many times before audiences get weary of the repetition. Introducing them elsewhere allowed Marvel to gauge the feedback (not to mention sales) before committing resources to a full-blown franchise. It was risky and expensive to add a new comic about an untested hero to the company’s line. This way of introducing characters made a lot of sense in comics. The Silver Surfer soared through the cosmos for the first time in Fantastic Four #48. While Captain America debuted back in 1941 in the pages of Captain America Comics #1, his introduction into the Marvel Universe came as a special guest star in The Avengers #4, when the Avengers thawed him out of an iceberg he’d been trapped in since the end of World War II. The Punisher first debuted in the pages of The Amazing Spider-Man #129. Wolverine’s first appearance came in The Incredible Hulk #181. most of the company’s most famous characters debuted as guest stars in other series. After the first generation of Marvel’s Silver Age superheroes - the Fantastic Four, the Hulk, Spider-Man, Thor, etc.