You can take a look at the fully functional Aetheria Game Engine, for inspiration if nothing else. It’s Java too, under BSD license.
I’m sure Al-Khwarizmi (the author) is open to colaborations, from english-speaking people in particular. He’s very nice to work with, by the way.
https://intfiction.org/t/some-general-ill-informed-questions-about-if-directions/2625/77
https://intfiction.org/t/if-in-another-language/6393/7
https://intfiction.org/t/mud-maker/6235/19
https://intfiction.org/t/rpg-flavoured-parser-thing-a-penny-for-your-thoughts/5265/40
https://intfiction.org/t/ideal-language-for-playing-interactive-fiction/6133/11
Hope this helps.