I really liked them using an established character as the antagonist. I know it's not the first time they've had a character hop over the hero/villain line, it's not even the first time it's happened to Wanda, but so many of the MCU films and TV shows have a big bad we've never met before, and a lot of the time they don't survive beyond the end of it either. They need to get better at writing ways villains are defeated without being killed, but they can also do stuff like this. A big part of comic fandom seems to be people discussing how a fight would go between this character and that character and so many Marvel heroes have potential to be dickheads and act as the villain in individual stories.
Something that has potential to bother me is the lack of clarity on the rules of the Multiverses. I know they've said 616 Reed Richards is yet to be cast, but I'm not sure how that works. Stephen Strange in every other universe we saw looks like Cumberbatch, Wanda in every universe we saw looks like Elizabeth Olsen, Kang always looks like Jonathan Majors, but we know not every Peter Parker looks the same, and not ever Loki looks the same.