But doesn't the ecosystem around the editor depend on the editor being, at its core, Vim? https://neovim.io/doc/user/lua-guide/#lua-guide-api describes "three different layers": basic examples like https://neovim.io/doc/user/lua/#watch-file call into the Nvim layer.

I think the idea's good, but I don't think you could modify Neovim to support it while retaining the ecosystem – not without also modifying much of the ecosystem, at which point it may be easier to do a straight port to a different editor, or construct a compatibility layer (essentially reimplementing enough of Neovim in the new editor that the ecosystem still works).