Hi! I needed to share this theory properly, so here comes my first post on the site! I have thought a lot about this and would love your input!
I just read that in official Canon, Nightsister Acolyte was a rank within the Nightsister system. So I decided to make some adjustments on my earlier theory on the same topic and post again.
So I really think that considering the time the show is set to (the last years of the High Republic), the red theme on the title, and confirmed female centered characters, it will take place on Dathomir focusing on the Nightsisters rise to their power or discovery of their power drawn from the core of Dathomir. I also think it makes sense that we will get to see more of the Nightsisters after their name drop in Book of Boba, and Merrins appearance in Fallen Order, as we will most likely see more of them in a rumored FO sequel.
In legends a female Jedi named Allya was sent to Dathomir into exile, by the Jedi council. It was said she later trained women on Dathomir to learn to use the FORCE, and learn to tame the native Rancor on the planet, and these women are the ones that later would be known as the Witches of Dathomir. And a branch off from these witches became the Nightsisters, a group more investing in learning dark side spells and magic. Maybe Allya will play a part in it? This event with Allya was originally in legends said to have happened 300 years before the High republic, but maybe we will see some of this in flash backs, force echoes or hear stories of the first Witches of Dathomir.
As it is set to be in the High Republic, lasted from about 82-300 years before A New Hope there is little chance to see young Palpatine considering he was 2 years old when High republic era ends, and wasn't introduced to the force before he was 17.
It is however possible we might see Palpatines master, Darth Plagueis and his master, Darth Tenebrous. Perhaps as antogonists/villains of the series. Perhaps they are visiting Dathomir after hearing of the great powers within the planet's core, seeking to gain control over it. Or maybe they will attempt to train them or create an alliance with the Nightsisters.
The sith at that time were hidden from the public world, but were seeking to gain power and allies in the dark corners of the galaxy, who could help them reach a power to be great enogh to some day topple the Jedi Order. Plagueis was also very intrigued by all kinds of different force and dark side power users, and were also known to capture creatures and people to perform all sorts of tests and experiments on them. Perhaps Plagueis will kidnap Nightsisters, and we will see how the witches attempt to revenge or recue their captive sisters.
We may also see a young Talzin. If I understood the timeline correct she were not yet a Mother among the witches at the time. A Nightsister called Zalem was mother before Talzin. I think it would be really interesting to see some powerconflict within the order of the Witches with Talzin being a part of it. Could Talzin be the Acolyte protagonist?
There are plenty of possiblities for what we will see, but I do think it will happen on Dathomir, and I really hope we will see Plagueis in it. I know many think it will be about the sith at the time, but to me that seems unlikely considered it will be female centered, is set to the end of the High republic and the sith known at that time were male (Plagueis and Tenebrous).
Please tell me what you all think!