As you know, WordPress allows you to hook into existing hooks/actions to extend functionality.
The disadvantage here is that you need to modify your theme’s functions.php for this (or create a child theme and modify it there).
EME now allows you to create separate PHP files in wp-content/uploads/events-made-easy/includes . If those files start with “eme_” and end with the “.php” extension, they will get included by EME.
A practical usage is already the creation of new payment gateways, see this file: eme_extra_pg