I have a couple of suggestions for content you can offer for a fee.
First, add a section called “Keeping up with the news” that includes terms commonly found in news stories, including country names and terms like “battle”, “explosion”, “treaty”, etc.
Second, add discrete sections on specific areas of interest: business vocabulary, medical vocabulary, scientific vocabulary, engineering vocabulary, finance vocabulary, animals (e.g., dog breeds), plant names (e.g., pine tree, maple tree).
Third, offer an option that includes image mnemonics: pictures with and without spoken or printed prompts.
I’d pay extra for all of these options.