I think the very best thing that can be added to Space Engine that can start very simple and shouldn't (in theory) be a huge project, would be simple base building. Something that can be expanded on and even handed largely over to a modding community. It would give us something to do with all the planets and stars! Other things involve so many intermingling concepts and projects and so much time, whereas a simple base/colony building and sharing mechanic would provide immediate stuff to do, immediate fun and would fit right in with the whole idea of Space Engine.
Don't get me wrong, I have a HUGE wish list and this isn't even at the top of my wish list, but when thinking what this one-man project could do to add some gameplay quickly, this is the first thing that comes to mind.