The Godfather
The Terminator
Halloween (original and 2018 sequel)
All of these told pretty killer stories that needed no third chapter. In the case of Terminator, they tried again and again, then even James Cameron came in and finally tried himself, and it didn't work. I honestly don't think there is an idea for a third Terminator film out there in the ether that works. It's just...Well, a perfect duology.
Anyone else?