I don't spend a lot of time on internet forums, but I did darken your door here about five years ago with this posting about a vacuum tube theremin I had built. I recently finished designing and building a digital theremin that has worked out really well. (Apologies to the traditionalists among you; I wanted a challenge at the opposite end of the technology spectrum. 
) The new theremin is based on the STM32F746 microcontroller, and it is built from off-the-shelf components, except for some hand-wound inductors and of course the antennas and cabinet. Here is an article describing it, complete with a sound clip:
A Digital Theremin Built With Off-the-shelf Components
At some point in the future, I hope to publish more detailed information (schematics, PCB layout, software, etc.), but first I have a lot of work to do to organize it well enough so that folks don't need a lot of technical support to build upon the project. Meanwhile, I hope you'll find the article interesting.