designing some sensors for my pi.

[lilzz]

On this circuit, the thermistor sensor using a pull-up on + side of opamp, whereas position sensor using pulldown on + side of opamp.

Since opamp is just a voltage follower, so why not just use pull-up for both or pull down for both.
Why need one for pull-down and another one pull-up?

[totoharibo]

On this circuit, the thermistor sensor using a pull-up on + side of opamp, whereas position sensor using pulldown on + side of opamp.

Since opamp is just a voltage follower, so why not just use pull-up for both or pull down for both.
Why need one for pull-down and another one pull-up?

your sensors have 2 wires.
I see only one on your drawing.
Where goes the second ?

On the other way : GPIO inputs are digital. Output of your AOs are analogous. That will never work you need an analog-digital converter.