The Terminator truly is a standalone concept, but James Cameron was able to create a great sequel by twisting the conceit of the first one and expanding its scope. They're both great films.
Frankly, without Cameron, the franchise is worthless. Hollywood has tried three times without him to make a Terminator film. It hasn't worked and Cameron is not likely to return to the IP.
Hollywood needs to let it go.
This, this, this. The Terminator was a standalone film, and a tremendous one at that. It's a truly fantastic character driven piece of science fiction. It didn't need a sequel, and yet, Cameron came back and delivered a remarkable one that worked perfectly with what the first set up.
The only hope I have for the franchise outside of Cameron is for another filmmaker with vision to grab hold of the reins and do something interesting, and not necessarily with a gigantic budget. I'd be interested if someone like Rian Johnson were to give it a go without studio intervention. The need for a massive, multi-film, sprawling epic is poison - find a good story to tell, and tell it.