Thanks again! Ultimately my goal is to have a better understanding about the limits of how to alter the core features of moodle without having to fork the git repo, i.e. purely with plugins. That's why I was interested in modifying directly whatever the "main_content" method of the core_renderer class produces, via a mustache template.
You are right that the "Create a new account" button is readily available, but I was testing if it would be plausible to create an super simplified "create new account" form directly to the login screen. This would mean that, for example, just a gotcha and an email address.