why do you recommend Olivier Messiaen?
Khachaturian Piano Concerto, second movement. Big Theremin moment, right? Always sounded a little silly to me -- rather dated and reminiscent of B-Movie soundtracks for schlocky horror films.
He wrote a lot of works that exclusively feature the ondes martenot, a close relative of the theremin. I don't recall any high profile composers that wrote a lot for the theremin, so I recommended him.
Here is the flexatone in a humorous take on the Titanic theme:I love this instrument. I'm gonna go listen to a few pieces that use it now.