Using the Hornbostel-Sachs1 system, there are five main types of instruments. For this research we're not trying to create a whole new category, but neither do we want to recreate one of these principles.
1 - Idiophones:
Instruments which produce sound by vibrating themselves;
2 - Membranophones:
Instruments which produce sound by a vibrating membrane;
3 - Chordophones:
Instruments which produce sound by vibrating strings;
4 - Aerophones:
Instruments which produce sound by vibrating columns of air;
5 - Electrophones:
Instruments which produce sound electronically.
Somewhat simpler explanation
Hydraulophones - Makes sound from a liquid state
Plasmaphones - Sound from plasmatious gas
Non-electrophonic quintephones - non electronic compotational devices which produce sound
An important part of making an instrument is choosing the range. As a general rule of thumb, larger instruments will be better at producing low tones, while small instruments produce high tones.
Classical theory divides instruments in five different ranges, from high to low: Soprano, Alto, Tenor, Baritone, Bass. Each corresponding to a 3 octave range from 30Hz to 4000Hz.