I've been using 555's for A to D converters for over 30 years!
Just use the 555 in monostable mode, with your resistive sensor as the timing element, trigger the 555, count till it triggers.
You can do the same thing with a capacitive sensor.
Every joystick interface that wasn't a switched stick, from the Apple 2 right up to the PC works this way.
Only bad thing is you have to wait, unless it's a bit of hardware doing the counting.
The dual, quad and octal 555 variants can be tricky, they tend to interact with each other, so you gotta decouple the power rails pretty well.