You missed https://github.com/maralla/completor.vim https://github.com/maralla/completor.vim - one of the first async ones targeting vim8, later gaining neovim

You missed https://github.com/maralla/completor.vim https://github.com/maralla/completor.vim - one of the first async ones targeting vim8, later gaining neovim support. It doesn't look like it's easily extensible though, aside from using omnicompletion, which negates the async benefits. Note that mucomplete isn't async, it's intended to be simple.