Marvel Studios officially launched Phase 4 with WandaVision, which is its most unique project to date. Aside from looking and feeling nothing like the usual superhero project, at least at first, it's also the franchise's Disney+ debut. Still, the show is very much tied to the bigger universe that it belonged in, and given that, it's curious why no one else from the MCU popped up in Westview even just for a brief cameo.
Marvel Studios took its time progressing the story of WandaVision, building the mystery week-after-week. It was only in the penultimate episode, "Previously On" that it was revealed what exactly brought on the Maximoff Anomaly in relation to everything else that's known about the MCU at that time. Given this character-driven storytelling, there were few opportunities to bring in someone from the greater franchise to cameo in a way that doesn't feel shoehorned, nevertheless, the openings, particularly during the finale, as well as, in WandaVision's post-credits scenes were there and the show didn't take advantage of them.
Confirmed to connect narratively to Reed Richards would debut following Monica Rambeau's aerospace engineer tease, setting up the imminent arrival of the Fantastic Four. However, WandaVision opted to not take advantage of these opportunities and it made sense considering the show's commitment to keeping the focus on Wanda. While Fury or Doctor Strange's potential appearance would've been brief, they could still distract from its primary narrative which was to see Wanda dealing with grief and how this experience results in the official birth of Scarlet Witch in the MCU.
This was exactly what happened in WandaVision's ending bringing in any well-established character from the MCU at the very last minute would more likely have been most viewers' takeaway. Instead of pondering about what happened in the series, discussions will be dominated by talks about those surprise appearances and what they mean for the future of the franchise.
Granted that interconnected storytelling lends itself to these kinds of discourse, it's nice to have something like WandaVision in the MCU which is mostly contained. Marvel Studios is not immune to this misstep, however. MCU's future.