So, as everyone has seen, the new WR trailer has one of the characters screaming loudly in pain/desperation/anger (who knows). Most people I've seen were theorizing that the scream was Sayaka, Kyoko, Homura, or one of the new girls, but this tweet shows that the scream is also likely to be from Aoi Yuki, based on her screams as Hibiki in Symphogear.
But I think we can all agree that, even if it might sound like Aoi Yuki, this doesn't sound anything like how Madoka would scream though, right?
This brings me back to some of my theories. The two other new characters have had their voice actresses' announced, but not this mysterious girl.
I've seen and/or have come up with 1000 guesses of who this girl is, but my number 1 theory for a while is that this girl is The Law of Cycles, who has taken a physical form as another girl after being separated from Madoka.
If this is the case, it would make sense for her to be also voiced by Aoi Yuki (which I've also theorized a while ago), since she and Madoka are two halfs of the same entity. In the very first trailer we've also seen this girl with a bow, and her magic being red with a little pink, which points to her being somehow connected to Madoka and Homura (though her design also seems to have some elements from Mami, and maybe others in the quintet).
My other main guess which most people in the fandom have also thought of is that she's Walpurgisnacht as a magical girl, but from supposed leaks of Madoka interacting with "Wasu" in the script and some imagery in the latest trailer, it's very likely that the human form of Walpurgisnacht (if she even has one) is the "fake Homura" rather than this girl.