I saw that the premium edition of one of the volumes of Unwanted Undead has a glossary as a bonus item. I think that the idea of a glossary, or definitions of LN/Japanese terminology has come up before. I'd like to see a wiki or index or something- either on the website or as links within prepubs of these. I think the wiki could also have articles that contribute to the background of stories. Information on locales, aspects of Japanese culture, etc. that contribute to plots of stories. A repository of relevant editor/translator notes, recipes of foods that are relevant (and what some of the ingredients are)
This would be a nice reference to have even if making it publicly available could reduce the value of some premium editions.
However, it might be even better to find some existing English-language wiki that could be expanded to include the LN material, and link to that instead of reinventing the wheel. (I have no idea whether such a wiki exists.)
Who knows, other publishers might even be willing to participate in creating one big wiki instead of many little ones.