This is so, so close to allow me to run a full local vim (with the necessary external tools, like python and git). The only thing I found that was a dealbreaker was the lack of unicode support (https://github.com/tbodt/ish/issues/28), and not being able to map caps-lock to escape.

For now, Nicolas Holzschuch's fork of iVim (https://github.com/holzschu/iVim) is still by far the best solution, but iSH definitely has the potential to take over.