See GSoC 2017 Project Brainstorming on OpenMRS Talk for additional project ideas in the pipeline.
In general, Mentors are community volunteers who are willing to give a student a great experience in open source development.
Backup mentors have no explicit expectations beyond staying abreast of a project in case they need to fill in or take over for the primary mentor in case of an unforeseen circumstance. While we generally avoid anyone being a primary mentor for more than one project, we do allow primary mentors to serve as a backup mentor for one other project, since the need for backup mentors to take over the role of primary mentor for a project is very rare.