I was surfing around and ran into Nyquist, A Sound Synthesis and
Composition Language. I'm wondering if LMMS could support Nyquist
instruments... I was thinking about a plugin with a simple prompt for
the LISP code that would return a sound. LMMS could provide the
environment by setting the Nyquist's global variables *loud*,
*transpose* and *sustain*. A prompt would be a good way to go because
you could just write (load "myinstrument.lsp") to use external files.
Audacity is doing it too: