Let's add UI to install WordPress Plugins on playground.wordpress.net. It may or may not be similar to the one we already have for selecting specific PHP and WP versions (thanks to @elchead):

Technically, that UI only needs to reload the page with one more plugin parameters, e.g. &plugin=coblocks&plugin=friends. The challenge here is creating an ergonomic user experience.

A good starting point is this file rendering the entire Playground.wordpress.net website:

Done is:

• A UI control for adding plugins exists
• It works well on mobile