I recently gave an app called LingoDeer a try. While I found that it wasn’t for me it had something that I found interesting. It had a book icon that helped explain some grammar.
I was thinking how the Bunpo app has good grammar and examples for the JLPT. You could add example sentences with grammar based on the different JLPT levels. Your could and add the book icon to those sentences. That book icon would explain the grammar related to the sentence. That way users could learn grammar as they use sentences to study.