What would it take to make Craft composer based? Since there is a free edition and the plugin architecture looks relatively simple, I'd like to know if anyone has an idea of how this could work. Are there any plans for it?
I'm not 100% sure what you're looking for, but Craft does use Composer to manage its own dependencies.

If you're referring to using Composer in your plugin, it's as simple as creating your composer.json file in your plugin's root folder and running composer update.