SPOILER WARNING!!!
This entire theory is based on the Puella Magi Madoka Magica Concept Movie (Which you can see here: https://youtu.be/xRq01091Fl4) so
this is all one wild guess, as we don’t know if it’s the concept for Walpurgis no Kaiten yet, but hey, why not make a theory anyways?
I believe that Homura Akemi and Walpurgisnacht will be the main antagonists, but not in the way you think for Walpurgisnacht.
One day, while watching the Madoka Magica Concept Movie for the first time, a certain image caught my attention. This one:
Now then, if you pay attention to the design to the design of this strange creature you’ll notice some very important things.
Firstly is the strange tendrils. This obviously takes inspiration from Homura’s Doppel in Magia Record, but in place of the mini Homuras, are minor versions of her minions.
Nextly, are the wings near Homura’s head. If you remember, when Madoka became Madokami (That’s just what I call Ultimate Madoka), she had transparent wings behind her. However, besides being Homura’s purpose in life, what does Madoka have to do with Homura? In Puella Magi Madoka Magica: Rebellion, Homura “took pity” on Madoka and split her in two. This gave birth to Devil Homura, who also had these wings, except Devil Homura’s were black. The dress that this thing wears might also take inspiration from the one Devil Homura wears, but this is a tiny stretch.
Thirdly, there is a great rainbow ring behind what I assume is Homura Akemi. This ring shares some resemblance to the ring often seen behind Walpurgisnacht, who was the most powerful Witch and the only Witch capable of rivaling Homura. Walpurgisnacht was also the main reason for Homura becoming a Magical Girl, Madoka taking the “Ultimate Sacrifice,” and for Madoka becoming a Magical Girl as well in the current timeline.
Finally, if you look behind the creature, you will notice that they’re in some sort of colosseum. This is similar to, or may even be, the same colosseum that is the center of Homulily’s Labyrinth in Rebellion. If you recall, when Homura became Homulily, she was in a giant colosseum which was then put on some sort of arch for Homulily to walk on.

But what about Walpurgisnacht? What’s their role in this? I’m willing to bet that Homura either fuses with Walpurgisnahct through her powers, or she absorbs it’s power. So then, this leaves one main question. Why? Why would Homura Akemi do this? Well, the answer is that she’s trying to protect Madoka, from herself. What this means is that even though Homura split Madoka in half, there’s still a half of her still out there. That half being inside of Sayaka Miki and Nagisa Momoe. There’s proof of this when Sayaka says at the end of Rebellion after almost summoning Oktavia von Sekendroff, “I…I think…I remember being a part of something, some being, much bigger than this world. But I can’t remember what it was anymore. All I know is that I’m not supposed to be here.” This means that while she does remember the Law of Cycles, her memory is extremely foggy, and she could possibly regain it if the Concept Movie is true. So then, Homura must do everything in her power to destroy Sayaka and Nagisa, to prevent Madoka from becoming whole and regaining her memories. Why? So that she doesn’t become Madokami.
Thanks for reading this! But again, this is all one giant theory, a film theory, and with that I bid you adieu. Have a great day and great night! :)