I think that the utility of multipass for spawning windows server / windows whatever (build machines) or macOS (on Mac hardware) images is obvious. I would up that with allowing some of the BSD family to exist in the ecosystem, in case you want to compile and test your code there.
Is that within scope of the project?
It’s one thing to spawn it, another to be able to do anything with it.
multipass launch is just the first thing you do to an instance, being able to
transfer is a much trickier task.
Multipass relies on cloud-init to bootstrap the instance in a way that we can then manage it. While plausible with BSD, it’s less so with Windows or macOS, which would require custom machinery to integrate with.