Kind of hard to code I think, but maybe you can have an option to make an instrument that simulates how an average man or woman pronounces digraphs and letters, so that we can somehow listen to a person singing in a Sequence, plus, there are many Friday Night Funkin' covers and choir instrument requests.
Does an OS Vocaloid exist already?
Its hard to do that even in surge synths, there is a reason hatsune miku is expensive. Try Liams FFT trick
For some actual singing, I recommend plugging in a MIDI keyboard to Google's Blob Opera Website, it takes in MIDI inputs and outputs the blobs. The blob voices are actually real recordings of real vocalists.
This doesn't really have anything to do with OS, but I thought it kinda pertained to what you wanted, and I thought I'd mention it.
https://artsandculture.google.com/experi...=en&cp=e30