A simple example of how composable custom Phoenix generators might work for discussion purposes. Tested in the sense that I followed the instructions and I could use Tailwind and Alpine in the app afterwards.
Install locally with mix archive.install github srowley/phx_add.
mix phx.new my_app --live
cd my_app
mix phx.add.tailwind
mix phx.add.alpine
Note that the Alpine generator assumes the new app is a Live View app; it would be easy to fix so that it worked for a regular app.