Terminator: Dark Fate may be making some major changes to the Terminator mythology as fans know it. Dark Fate is produced by James Cameron, who directed The Terminator and T2: Judgment Day. The film is intended as a direct sequel to T2: Judgment Day, ignoring the sequels Terminator 3: Rise of the Machines, Terminator Salvation, and Terminator: Genisys.
Fans will remember that, in the original film, the synthetic intelligence network known as Skynet sent the T-800 Terminator (Arnold Schwarzenegger) back in to eliminate Sarah Conner (Linda Hamilton), the soon-to-be mother of John Connor, the human savior in the coming war between man and machine. In T2: Judgment Day, Skynet sent a T-1000 back in time to kill John Connor, but the human resistance sent back a T-800 to protect him. Together, Sarah, John and the T-800 assaulted and destroyed Cyberdyne, the company that creates Skynet. This prevents Judgement Day from happening (at least, that's what it is implied. Terminator 3: Rise of the Machines reveals that the US government had a backup Skynet in the works leading to a backup Judgement Day but, as mentioned, James Cameron does not care).
So how does Terminator: Dark Fate change our understanding of these events? We don't know for sure, but a rumor may spoiler a big twist. SPOILERS follow.
According to Heroic Hollywood, Sarah and John Connor's mission to prevent the rise of Skynet was as success. There is no more Skynet in Terminator: Dark Fate. But that means Dark Fate takes place on an alternate timeline after the events of T2: Judgment Day. After all, without the rise of Skynet, who sends the Terminators back in time?
But a new threat has risen to take Skynet's place. This threat is known as Legion. We don't know much about Legion's nature or how it is different from Skynet, but it seems it threatens the future of humanity in a similar way. That means a new war for the fate of humanity is on the way, but this new war needs a new savior. Daniella "Dani" Ramos, played by Natalia Reyes, is that savior. Rev-9, an advanced Terminator prototype played by Gabriel Luna, is sent back in time to eliminate Dani. Someone — presumably the human resistance fighting Legion — sends the cyborg assassin Grace, played by MacKenzie Davis, to protect Dani, setting up a classic Terminator conflict. Sarah Connor calls on her old friend the T-800 to get involved and help protect Dani.