Since we know main writing is done.. (barring continuous editing)
And we know that Y2 has existed in a playable state for years, now.
I am suspecting that most of the remaining work revolves around making sure all the massive tables of information reference each other properly. And possibly optimizing said tables since they are the heart of the game and it would be just a bit annoying to have to wait for all those obscene files to load. There's bound to be other things, but that would take the longest for a small team to accomplish. Especially a small team working on it as a hobby.