Imagine that you don't actually know what your full end-state will look like, sort of like what you are doing now with Spec Kit.
I started doing my own product development with a long backlog.md file and then started breaking that out into separate sections by major and minor version. Then I started adding feature-plan-vX.X.X.md for each feature iteration I was building.
I think it would be really useful to be able to apply this concept within Spec Kit so that individuals and/or teams could build out specs by feature / version. Agile feature development and planning and release planning tends to change over time based on customer feedback.

Hope you will consider adding this capability: I hope to use it in my current project. :-)

Thanks,
Derek (softwaresalt)

PS: You seem to be thinking some of the same things I am in my article on Chimera Coding.