Adding a NTC thermistor inline with a pot
You can make your circuit work with an NTC.
I was unable to get the 555 component to work in the simulator, so I built just the threshold components using an Op-amp. It was fun to get going in Circuit Lab. 

The box on the left shows the components for charge/discharge of the capacitor. 
R2 and R3 represent what you originally showed as a potentiometer, but in this case it's easier to test with just two resistors in the simulator.
If you make R3 the NTC, then the duty cycle increases with an increase in temperature.
If you made R2 the NTC the duty cycle would reduce with an increase in temperature.

The major problem with circuits such as this is that as the duty cycle is increased the frequency changes too. While the fan should be insensitive to the frequency, it's still a pain.    

You also need to consider that this isn't exactly PID, so it won't ever increase the fan speed to overcome a temperature increase. So your PC/device will increase in temperature, you cannot maintain a fixed temperature. 







simulate this circuit &ndash; Schematic created using CircuitLab

Waveforms from the simulator: