The following contains spoilers for the first seven episodes of WandaVision.
A new phase of the MCU has been ushered in with the release of WandaVision, and it's certainly something different from what Marvel has done before. Even if WandaVision wasn't the intended beginning of this new era, it's an interesting jumping-off point because it seems like it might be setting the tone for what's to come in this next phase of the MCU.
With the conclusion of the Infinity Saga in 2019 with Avengers: Endgame, Marvel needed to figure out what direction they were going to take next. What could they possibly do to follow up such an epic superhero team-up movie, and what kinds of threats could they pull from the comics to the screen that are even bigger than Thanos? WandaVision seems to be implying that the threats in the future of the MCU aren't going to come from space, as they have in the past, but are going to be of a more magical nature.
The MCU has always been very science-based, with magic only appearing occasionally (for instance, in Doctor Strange). However, WandaVision, specifically most recently in episode 7, has introduced magic as something that is going to be coming to the forefront of the MCU. Wanda's abilities have obviously been front and center over the course of the series, but in the past couple of episodes, we've seen her son Billy manifest very similar powers, and the revelation at the end of episode 7 that "Agnes" is actually Agatha Harkness brings even more magic into the universe.
In the comics, Agatha Harkness is a witch whose story becomes intertwined with Wanda's when Wanda uses Agatha's magic to create her twin sons (since she could not have biological children with Vision due to him not being human). Her story is also intertwined with the demon Mephisto, because it turns out that the magic the twins were made of was part of Mephisto's soul, and when he "unmakes" the twins in order to reabsorb that magic, Agatha wipes Wanda's memory of the events to spare her from that pain.
This is important because Agatha appearing in the show with her magical basement lair is the biggest hint yet that magic has come to the MCU in a big way, and that Mephisto himself might even make an appearance. If the baddies in WandaVision are beings who use magic, and if WandaVision is set to tie directly into Doctor Strange in the Multiverse of Madness, it's safe to assume that the biggest threat to the world in this new phase of the MCU might be magical.
Rather than space aliens invading Earth, this next chapter of the MCU might be heavily influenced by magic, and might introduce or bring back villains who are magic users. It would be an interesting direction for Marvel to take after the past decade of films, because magic opens up a lot of possibilities for fun things that the MCU could do, and opens the door for a lot of comic characters who might make an appearance in the future.
Even though a lot of MCU characters have superhuman abilities, it's usually explained as something chemical or biological in their bodies, and science is used to ground those powers and make them feel more "believable." However, there have been some characters who are magic users or who dabble in sorcery in the past in the MCU - Loki, Doctor Strange, even Wanda's unspecified powers have always been implied to be of a more magical variety.
Bringing in magic as a main plot point could allow the MCU to bring in characters like Morgan Le Fay, Enchantress, Doctor Doom, or as many WandaVision fans have been speculating, Mephisto. Quicksilver from the Fox X-Men universe making an appearance in WandaVision is also a hint that the MCU might finally be bringing in mutants, which could introduce another slew of magic-using characters.
For fans who love these kinds of magical storylines, it could be very exciting that Marvel appears to be taking this route. Magic is intriguing for a lot of audiences, and might be easier to understand or become familiar with than a lot of the space and science storylines that have been staples of the Marvel Cinematic Universe up until this point. In a lot of ways, it seems like Marvel is doing things a lot differently in Phase 4, and it might be a much-needed refresh for the franchise to make these changes, and to really focus on something that they haven't had a lot of time to explore in the past.
Though it's all speculation at this point, there's a good chance that the MCU might make magic and magical enemies the next big thing that the heroes have to deal with, and cosmic threats might take a bit of a backseat, at least at the beginning of Phase 4. It will be interesting to see what Marvel has planned for this new chapter in the franchise, and how they intend to have it live up to the epic story that they gave fans in Phase 3.