Here's a piece on Salon's Audiofile about musical instruments of the past that never quite made it. It also wonders about the musical instruments of the future (check out the Hyperviolin for a musical instrument that looks and sounds like it came straight out of Soderbergh's Solaris).
Unmentioned, however, is the elephant in the room of music futurama: real time electronic signal processing. Well, okay, not really. The elephant in the room is the computer. There's nothing (besides look all cool and shit) the Hyperviolin can do that couldn't be done with a regular violin, electronics, and a laptop (predictably, Mac makes an application called AU Lab that will facilitate this. Also predictably, they give it away for free in their Audio SDK). Don't get me wrong, this is craftsmanship and skill and it's hard to pull off (Fronesis will, on a related topic, argue vociferously that a certain guitarist for a certain band he likes, while not as technically adept at the guitar itself as other professionals, is highly adept at the signal post-processing, and in turn that the point of a musical instrument is the beauty of the sound you produce with it, and that signal processing is an integral part of modern musical instrumentation. Ergo The Edge is a genius). My argument here is that these new musical instruments are not new--maybe the physical configurations of electronics are, but the instruments can (or do) exist already in the combining of existing equipment.
I'm also not one to scoff at a laptop being called a musical instrument. Playing one is a skill and it requires practice--I have tried making songs out of sampling, looping, and software instruments, and I sucked at it. I sucked at it so much that it became immediately apparent that I'd need lessons in order to ever get good at it. I am, on the other hand, ignoring this instrument at the peril of my musical career: on MacIdol, the overwhelming majority of the songs posted, and the overwhelming majority of songs listened to, are (for desperate want of a better term) instrumental electronica. Like everyone else making sounds, I'm looking for that next thing, Big and New; I'd bet my money that it's going to come out of somebody's CPU somewhere.