Well, here are the properties that I would desire in a Theremin in order of preference:
1) Playability
For me, the playability would be must important -- that is, the circuits are stable -- the pitch antenna has good horizontal linearity and is not greatly sensitive to up down motions (that is, motion that is parallel to the antenna).
The volume antenna would allow me to mute the sound without actually having to touch the volume antenna. Also, the volume antenna would have variable sensitivity.
For me, beautiful tone and styling would not be a substitute for playabilty.
2) Clean tone
This translates to excellent shielding of the components from electrical interference and a good signal to noise ratio.
3) Musically interesting tone and tone controls
Interesting tone color along with the ability to play the "native" heterodyne tone without alteration. Wave shapers or other methods to color the tone would be welcome additions, of course. Make the tone controls controllable from foot pedals and/or MIDI CC controller messages.
4) Adjustable height
Should have a standard mic stand or tripod mount and/or an easy way to adjust the height of the unit.
5) Portable
removable antennae, lightweight, easy to travel with.
6) Features
MIDI in and out, CV outs, preview output, Foot pedal controls to control the tone quality of the Theremin. A master volume control is a plus.
The Etherwave Pro has most of the qualities that I would desire in the ideal Theremin. If the Epro had MIDI and foot pedals to control the range and timbre switch, it would likely be the "ideal" instrument for me.