The new game uses the same underlying engine as Succubus Stories, so it will support modding, though I need to design, create, and expose specific APIs for modders, so the game will probably not support mods until later.
Neither this game nor Succubus Stories will ever be open source, but I get where you’re coming from. That said, the way things are done in the games themselves are not always good examples anyway, as I update the APIs over time and add new features, shortcuts for commonly used things, realize past me is an idiot and change the way things work, etc.
You actually can see the entire source code of the “Convent” mod, though, and I will probably make additional official mods in the future that will be source available if not open source. I’ll probably do something similar for Hex.
If you are interested in modding, you might want to consider joining the discord, as we have at least two community modders there who know the engine and tools pretty well, plus I’m of course also willing to help and give examples and advice.