Capacitance to Analog Voltage
Probably not. Heterodyning is an important part of how a Theremin works.

Keep in mind that the capacitance change caused by waving your hand near the antenna is a tiny fraction of the fixed/parasitic capacitance of the circuit overall, on the order of 0.2%.

The period of a 555 timer is directly proportional to capacitance, so a 0.2% change in capacitance results in a 0.2% change in period (or frequency). This would be difficult to detect and/or convert to a control voltage. If your 555 has a nominal period of 1 ms, you'll need to detect changes that cover a span of just 2 &micro;s.

On the other hand, if you have a 1 MHz LC-tuned RF oscillator, a 0.2% change in capacitance will shift its frequency by 0.1%, or 1000 Hz. If you then mix this oscillator with a second oscillator that's fixed at 1 MHz, you get a beat frequency that varies from 0 to 1000 Hz, a range that's much easier to work with.