There is only a rough draft, and no concrete plan. The idea was to extend the map a bit, add a few more encounters and scenes, a small progression and conclusion to the story. Essentially bring it to a close with a few additional features requested by some people.
But the main issue is the availability of CGs, as there are about 3-5 usable scenes from the original game that were not implemented, but after that, there won’t be anymore CGs to use.
This causes a bottleneck where we either create our own, have someone create them for free or pay someone to create them, and until that issue is solved, I don’t think there will be a definitive plan on the extension roadmap, because each option creates different levels of limitation.