eXist is participating in the Google Summer of Code again. Student applications can be submitted until March 26! The list of proposals along with the timeline can be found here:
Some of our project proposals may sound a bit ambiguous, but please don't be scared: we are here to help ;-) We chose those projects because they represent more or less separate work packages which can be handled without knowing the entire eXist code base.
The list includes, for example, a "remote debugging interface" for XQuery. We certainly don't expect someone to write a full-blown debugger. What we need is a well-defined debugging interface on the server and a simple command-line prototype, which demonstrates the interface.
Or take "index-support for order-by, distinct-values and aggregate functions": eXist's new indexing architecture makes it possible to implement the required functionality as an index plugin. Not too difficult. The challenge though will be to make it efficient.