Spoilers for The Terminator series
When James Cameron directed The Terminator back in 1984, it was only his second film- after Piranha Part Two: The Spawning (1981), and it ultimately changed his entire career, giving him the clout and respect to eventually direct the sequel to Alien- Aliens. He would eventually return to the Terminator universe with Terminator 2: Judgement Day (1991), a film that revolutionized special effects. And of course, he directed the two box office champs, Titanic (1997) and Avatar (2009), the latter of which was also revolutionary in regards to its special effects.
While Cameron is still very much focused on doing more Avatar films, this week he brought up the possibility of doing more Terminator films. Apparently he and producer David Ellison- who owns the rights to the franchise- have had discussions about a possible three movie story-line. Cameron hasn't been involved with the three previous Terminator films- Terminator 3: Rise of the Machines (2003), Terminator Salvation (2009) and Terminator: Genisys (2015) Cameron says was supportive of the Arnold Schwarzenegger led 3rd and 5th films- since him and Schwarzenegger have remained friends throughout the years- but adds they didn't work for him, "for various reasons."
For me I believe it's been difficult to make further Terminator films post the second film, arguably even the first film. This is because everything that occurs is on a loop- Kyle Reese will always be John Connor's father, Skynet can only ever be created because the Terminator came back. But coming back to T2, that's the saga's end. Sarah Connor and John prevented Judgement Day, Miles Dyson is dead. There was even an alternate ending set in the altered future where John had become a senator. In T3 all that was undone- Judgement Day is unavoidable. It was a bold way to end the film but the rest of the film- while decent- felt to similar to the previous films. Salvation was set in the future only briefly glimpsed in the first two films. It had potential but didn't capture the oppressive dread of that world. I admired the ideas explored in Genisys but it was too convoluted and fell in to the trap of having the plot revolve around preventing Judgement Day again.
The Terminator was never really designed to set up a franchise. As I mentioned before, the first film is a closed loop. And since time travel is a major factor in the franchise, things keep getting more confusing with each new film. If Cameron wants to direct or produce further Terminator films I think the best approach is to start with a new continuity. It makes things cleaner and less confusing.
I would also suggest not focusing on Sarah or John; create new human characters we can get invested in. I think you can still keep the basic premise: a machine comes back to kill a human and prevent the machines' defeat in the future. Bob "MovieBob" Chipman proposed ignoring the post-T2 films; the film would be set in a post-labour future where machines haven't turned against us. However, the machines have made certain people obsolete since they're not need for labour anymore. Someone would go back to prevent this, targeting Dyson's now grown daughter who has a prototype terminator who can protect her. Chipman also recommended going for a smaller scale, with which I strongly agree. I like the idea of the franchise going back its horror/sci-fi roots rather than attempting to repeat T2 again.
I don't know if Cameron will ever get around to doing these hypothetical Terminator films or when we'll see another film in the franchise. I'm always open-minded about reinventing a franchise- look at the new Planet of the Apes films. Until then- well, we'll always have the first two films.