By trying to cater to so many things you will drown this project and overwhelm yourself. I am not saying you should make it open source, but it may be a good idea to make this thing expandable so that people can develop their own solutions to their own problems in their workflows.
You are exactly right; I should keep that in mind as I gather general ideas. Expandability goes with modularity, which I am getting peoples' opinion on here.
It should have an airhorn button and a lowpass filter.
I can, with about 90% certainty, say that the Square will have an onboard LPF. Not too sure about the airhorn button though...
Or make it expandable, but only focus on your own aspect, and leave it as a platform to explore for others.
This is what I am going for and it has always been my philsophy.