I have the idea for a new instrument which doesn't work with wind/blowing, hitting, or keystroked, but rather pressure.
What factors should an instrument have? Is there a checklist that I can check the attributes of this instrument against?
As far as I'm concerned, there are two requirements for musical instruments:
I actually really like this question, though it is based on some premises that I believe are too easily jumped to. I use a generalized definition of music as "organized sound," therefore, the two criteria above are the only things necessary to satisfy an instrument to be used in making music.
Everything else is up to you, including what degree of control the player has over that sound. People have certainly invented instruments that have an element of random chance to them, and by no means do all instruments need to be pitch- and volume-controllable.
If you wish to refine your criteria to "a musical instrument useful for playing the majority of western music", then the requirements will change, but I'm not sure the answers will be as objective as one can be with this question.
(Edit: Note that I'm treating the question as "What characteristics would a new instrument ideally have?" rather than "What does my instrument minimally need to count as an instrument?" Both are IMO worth asking, although as I've spent time working with new instruments I notice a lot of them run into the same limitations. Hence my list here: )
You should be able to play it with your eyes closed.
It should be worthwhile becoming a virtuoso on the instrument.
There should be a difference between instrument-building and instrument-playing, and you shouldn't have to be an instrument-builder to be an instrument-player.
It should be able to change timbre, as well as volume and pitch, and changing one of those parameters should subtly change the others (this is more opinion that the others, but in my defense it's a carefully considered one)
If it uses power, it should be able to withstand power failure until the end of a song, and if it ever, ever, ever crashes it should reboot hands-free within one musical phrase (where both 'song' and 'phrase' length are determined by whatever style of music this instrument is used in)