This is pretty ambitious.

> True multi-threaded Elisp (planned) — real concurrency for the Lisp machine, not just cooperative threading

I'm really curious how this project plans to approach. The Emacs maintainers have discussed this topic but concluded that the subprocess support (together with cooperative threading) suffices for most use cases.

I think you'd have to design the entire Emacs around parallelism, like editor state (buffers, windows, the display engine) which is global and unsynchronized, data structures would have to be made thread-safe, etc.