Improving linearity would, of course, be nice, but I doubt would make the theremin that more popular, or that much easier to learn. Aerial fingering is hard, and with little reference to any other instrument; the theremin has a steep learning curve, with little instant gratification, especially in the first 6 months. A beginner is going to take two weeks to be able to play a song they pick, and when they almost can play it confidently, someone is going to come into the room, pick up an accordion, and in 20 seconds, be like "is that what you were trying to play?"
Most people buying instruments/learning a new instrument, are largely going to be 16-25 or so, its a lot of dedication to ask, the typical theremin's repertoire is not the various types of rock/pop/folk, they will typically want to play, playing with other people/live has a bunch of technical difficulties, a reasonable theremin setup is going to be at least 400$, most people I know who picked one up, give up quickly, or use it as a noise box or more in an experimental/noise genre. Which is what people see, and it how it gets its reputation. Largely, I dont think its popularity is due to a lack of a large number of world class classical thereminists, but more a lack a bunch of fairly competent ones they will see locally.
Most of these problems are sort of core to the concept of the instrument itself, and I can't at the moment imagine how an improved design will fix some of these problems. An increased repertoire might help, but the theremin is at a disadvantage at best, in playing many other genres technically, (well it could be fine, technically, playing many genres of folk, but, conceptually, well, its a bit odd.)
But if the theremin became popular, wouldn't it be self-defeating, as its main draw is the seeming magic of playing the air, if this becomes an everyday thing, what would be the appeal?
That all said, I could see designing a better theremin aimed at the noise/experimental/synth crowd (that is already familiar with the instrument, and is not trying to play tonally), at a lower price point than an etherwave, has a possibility of limited commercial success.
