10 Ways to Make an Awesome Terminator Sequel
9. Split the Narrative
Terminator isn't a war movie and never should be... but this still doesn't mean we don't want to see the future machine and human war explored in great detail, and we would still very much like to see what has become of John Connor in the future. But Terminator: Salvation lost much of the charm of the original films because it was unrelatable to most people. It followed people in an apocalyptic future battling for survival, as opposed to people in a still very human world, battling against a villain that shouldn't exist in the current time-line.
A way to deal with this would be to split the narrative. My suggestion would be to have an adult, future John Connor witness a "Terminator" being sent back in time to destroy his past, and then witness this time machine be completely destroyed, giving him no way to chase after the machine. His half of this movie would focus on a battle through the war zone in order to find a way he can head back in time and prevent this machine from messing up his future.
Whereas the other half of this film would focus on some very human protagonists (more on my suggestions for these later) trying to survive, much like the first three films, an attack from this "Terminator". Although, unlike the second and third, in this film they will be without the help of a robot savior... and unlike the first film, no one from the future would be on their side. That said, to combine the narratives, I would later have John Connor find himself a time machine, head back to the past, and help save the humans from destruction.