"I'm new here- I want to build my own Theremin but don't know where to begin." - gsmonks
What are your analog / digital electronics skill levels?
Picking a preexisting design to build is almost as difficult as designing it from the ground up due to (1) the plethora of marginal designs out there, and (2) the very low wheat to chaff ratio of basic information regarding Theremin operation.
If you would prefer to avoid all the hassle and have someone else build you one, I'll volunteer if you pay me $1 million up-front (negotiable) and don't mind waiting 10 years or so (a non-negotiable stars alignment interval). ;-)
"I had a Korg DSS-1 back in the 80's, too (the forerunner of the M-1 workstation and its later incarnations), and that technology too hasn't gone anywhere. I've been out of the loop for a long time, and what happens when I come back? I find everything just as I left it. The Korg products (eg: the M3R, M-series, including the M-88) and everything else is still stuck in the same rut."
Corporate electronic musical instruments, with the exception IMO of Roland in some specific cases, are stuck in the technological backwaters. EEs in these firms are currently busy tacking iPad interfaces onto essentially 1980's products. It must totally suck to be them.
"My quibble with the Theremin has always been its limited sound-palate. With the right electronics I can make one sound, not "like" a human voice but exactly like a human voice. My old Poly 6 wasn't a great synth by any means, but I used to create voices you couldn't tell from human. With the sampled voices of today, and the great range of expression available, there's no reason you can't use a Theremin as a controller and do some pretty amazing stuff."
I agree with you entirely here.
"There's also pitch-correction software out there that could make playing a Theremin perfectly in tune simple and easy."
Perhaps, but I haven't seen this to be the case in practice (vicariously via video so grain of salt). Strong, stepped chromatic pitch correction likely throws the player off, more of a hindrance than a help, but I could be wrong. Though pitch correction as an effect (~arpeggiation) seems to be useful.
Like Chobbs suggests, the Theremini may be your best bet, though few here have seen / played one yet. And FredM can set you on the right analog road if you decide to go that route.
Personally, I'm going down a mixed signal FPGA road with a two axis left hand and single axis right hand, highly responsive and hopefully intuitive visual "tuner", and DSP based voices.