So at the moment, there is lots of rumored shakeups at Lucasfilm, so the Acolyte series may never be made. I think it would be a better idea (story and financial) for Lucasfilm to make the High Republic capstone a standalone film instead of a series which would take up precious Disney+ scheduling time. The new movie would be in a similar structure to Mando: Take inspiration from Legends and preserve continuity while still being something new.

The film would start with Darth Tenebrous seeking a new apprentice after killing his master, and he finds two possible apprentices with similar potential in the Force as children. The first is a Muun (would become Darth Plageuis) and the second is a new character (likely satisfying the original Acolyte series' female lead). Tenebrous trains them both and often has them fight each other, but neither one succeeds in killing the other. The new character is better in a fight but Plageuis is just smart enough to evade her attempts of assassination.

So eventually the characters are 17-19 years old, and Tenebrous decides that enough is enough and that to satisfy the rule of 2 one of them must die. Tenebrous gives Plageuis and the new character two "different" missions that unknown to the apprentices will take them to the same place to kill each other. The apprentices set off on their missions.

The apprentices go to a system with two binary planets deep in the Outer Rim. Plageuis arrived first and looks around on one planet, which would create an opportunity to explore Plageuis' abilities when he raids a hidden Sith tomb for a Holocron which would lead him to the mission's "destination" but finds that it has little useful info, along with evidence that Tenebrous was setting him up (showcases Plageuis' improved mental ability--he would notice connections that others wouldn't).

The new character would visit the other planet, but would sense that she is being followed by an unknown Force-sensitive. Before the new character can continue the mission, the Force-sensitive captures her (perhaps with something as mundane as a Zygerrian energy net or as powerful as Force Sleep).

When the new character awakes, she finds herself in the company of one of the younger characters from the High Republic, who is now very old, maybe in their seventies or eighties. There are several options for who this character could be depending on what happens in the High Republic books. My pick would be Keeve Trennis, as a Jedi called Trennis was recounted by Yoda in Dooku: Jedi Lost as one who left the Order in an event which was "very sad."

Trennis tells the new character about the High Republic and the Jedi (this scene would explore what the Sith thought of the Jedi at the time). Trennis turns the new character away from her path through the Dark Side. The new character recounts a conversation with Plageuis where Plageuis suspected that Tenebrous and the Sith Master before had helped empower the Nihil from the shadows and keep the Jedi off-balance and on a path to their eventual collapse.

(This would give an explanation as to why the Jedi saw the Force in ways that were partially breaking of canon during the HR--they were being manipulated into seeing something that wasn't there and limiting their power--just like Palpatine did to hide himself in plain sight, the Jedi of the HR couldn't sense the Sith because the Sith determined what they could see through the Force)

Plageuis then confronts Tenebrous, who says that Plageuis is his new apprentice as he was the first to return and used his greatest strength, his mind, to figure out the plan. Tenebrous officially anoints him Darth Plageuis. Several years of training on both sides pass before the Sith lords find Trennis' hide-out.

There is a confrontation between the four characters, and a lightsaber duel begins. Plageuis and the new character duel separately. Trennis fights Tenebrous and stabs him through the stomach, but due to his species' physiology he will live. He activates the other half of Trennis' split saber and kills her with it. Tenebrous then watches Plageuis and the new character duel. Plageuis wins, but refuses to kill her, instead giving her the Vader-to-Luke speech about joining him to kill Tenebrous.

Instead, the new character leaps from a nearby cliff in a motion reminiscent of Luke's fall from the platform of Cloud City. The movie would end with Plageuis years later recounting the Tragedy of the New Character to a young Palpatine, even though her life or death was a mystery.