What @mksd mentioned (i.e., ticket curation) is very important and something we really need some assistance with. I’d also note that you’re welcome to create new tickets yourself.
Some more low-hanging fruit development-wise would be to add additional unit tests to core to improve code coverage (you can find the current state of code coverage here and also the gaps in coverage). Code coverage isn’t a perfect metric, but it’s quite clear that there are parts of the logic of the application that aren’t covered.
Another thing to do would just be to review the list of bugs in Sonar. Most of what I see are actually real bugs that should be fixed.
Neither of those two things are terribly exciting but they would help improve the OpenMRS product.