The real deal, an analogue Theremin is a beautiful and super fun instrument.
I know one or two Dutch artists who have made great subtle use of a Theremin in their work.
So this digital proposal -- also fun! Must resist… must resist… must resist…
PS I briefly had the Roland A49 controller which features a 'D-Beam', which effectively is a dual (I believe) optical sensor measuring differences in incoming light (caused by hand movement). It is a tad simple but certainly fun to experiment with. Its 0--127 (or vice versa) output can be mapped to any MIDI Cc#.