I think everyone has come to the conclusion that while MIDI/digital stuff is cool and all, nothing beats the sound of a real guitar/drum/orchestra. I don't think any new instruments will be invented. After all, what's the last instrument that was invented that's actually caught on? The electric guitar, which is really just the logical evolution of the acoustic guitar, which is hundreds of years old (The Theramin is used by like three bands, doesn't count). And distorted guitars have been around for over 40 years. Keyboards don't count as a new instrument as they're actually just a compilation of old instruments.