Right now the Prosemirror lib repo has a non-collaborative demo while the website repo has a fairly basic collaborative demo example.
I would pitch that a complex and reasonably complete server-side example would be really helpful to A) catch edges cases B) shape the boundary of ProseMirror itself and C) reduce recurring question volume.
If there’s room to collaborate on such a thing, then I don’t want to pre-emptively close that door because we didn’t get your input or invite you to collaborate. Or maybe you want to stay away from creating or endorsing anything resembling a complex example which would be completely understandable too.