>>The question is....why would you feel the need to attempt a piece that's beyond your ability or is unsuited to the instrument?
Because that is how we grow.
I absolutely love when I hear something done well on the theremin that I never thought was possible. Most of us would never have expected much out of a pitch only theremin till we heard Mable the Maytromin ensemble. Or considered as appropriate theremin stuff that members here like Kip, Peter, Jon, and Randy have pulled off. And if you fail, well good you learned a lot and did the work! The quote of the week from a dancer friend "Why be afraid of falling? The floor is not that far away."
As you get know know yourself as a theremin player, how to do the work and make your music happen, you just know what has/will work or what you feel compelled to play. At the very start, just beginning, yes, look at the advice thats been posted, it will help. Amethyste, you've worked past that point and the question may be how are you finding new pieces to play, new things to inspire you to work on them without it feeling like work? I love playing early and Baroque music, German Operetta, and heavy metal, some say it's not appropriate for theremin or any instrument or voice.
Ultimately, really?... doesn't it boil down to something you want to play so bad you *Play It*.
Thinking ahead about appropriateness, approval, anything other than making it happen is death to your creative endeavor.