Esperanto is a bit further down the list of the remaining top 50 languages to look at (T171652), but it jumped to the top because I had a developer ask me to recommend a project to work on, and I suggested an Esperanto stemmer. As a constructed language, Esperanto is very regular and well documented, so the barrier to implementing a stemmer is much lower for a non-speaker.
It's now available on GitHub; it's in Java and has a GPL3[†] license, so all of the technical details are in good shape. The next step is to get a review of the stemming quality from speakers.
[†] It was initially Apache 2.0, but the structure was heavily based on the Serbian stemmer, which is GPL3, so that seems more appropriate.