I have a theory about how a lot of this is going to lay down, if all these rumors are true. This is COMPLETE FANFICTION, but I wanted to piece this together to get an idea of what we may be seeing:
Daisy and John's characters will be the main characters of the film, or setup to be for the entire sequel trilogy with this movie. For some reason, they find Luke's robot hand and go on a quest to find him.
They probably encounter Daisy's character's father Han Solo and shenanigans.
Sometime in the movie they learn Luke was imprisoned for a long period of time, however they never find him in said prison when they go there.
It turns out sometime between Episode VI and VII, Luke was imprisoned for several years in terrible conditions, goes mad Gollum-style, and basically turns into this insane crazy person. He becomes malnourished, and eventually escapes the prison once he finally succumbs to the dark side of the force. Somewhere in here, he loses his robot hand in his attempt to escape hence why Daisy, John, and Han have it.
His body got so fucked up he had to get a complete cybernetic overhaul to even survive, much like his father before him. Perhaps it was his weakened state once he escaped prison, or maybe an accident happened causing his body to get fucked up.
He basically gets pissed off at the world and has nostalgia for his days that his father was alive, finds a piece of Vader's masks and relishes his father's legacy. In his own twisted way, he views Vader's final moments with him as a passing of the torch rather than what it really was (Vader becoming Anakin again). Because of this, he becomes a new Sith Lord .
Daisy and John finally encounters him, he unmasks revealing the truth that Luke was the bad guy all along. Daisy succumbs to the dark side.
The whole arc of the sequel trilogy will be getting your friends on track. John becomes the main character to help Daisy, Han and Leia become side characters cause they want to help Luke.