I would like to put the enrolment invitation on the list of "seeking for a new maintainer". It's a plugin for teachers to send manual invitations to enrol into your own course.
It has been listed as a popular plugin (https://techsupport.lambdasolutions.net/hc/en-us/articles/200427963-Popular-Moodle-Plugins), so you will code on something that people already like. You will find many fixes and improvements over different places to start working on it: https://github.com/ucla/moodle-enrol_invitation, https://github.com/mouneyrac/moodle-enrol_invitation/network or in the plugin page comments. I think it's a small plugin, perfect to start coding with Moodle