I would like to see something like a histogram for accuracy on grammar points. When I first learn a grammar point, I often fail it a bunch of times before it starts sticking. Then if I forget it, it will start to get worse again.
Another thing is I would like to see some kind of tracker of your reading progress. Maybe a way to self-grade how well you understood a passage (Likert scale?) or even the example sentences, so you could have a different measure of “I understand how to recall this grammar point” vs “I can read this.” That would be really useful to track, and would also give you feed back on how difficult users find the content to make sure it’s appropriate for the level of the grammar point and that you have enough variety in usage.
Lastly, and this is pie in the sky, but some kind of mock JLPT-type exam would be really cool. There’s already those big milestones of “you completed all the N5 grammar points,” but to go further and say you’ve really mastered those grammar points would be cool. I know that you could say “that’s the same as getting x SRS level” but it feels like that’s missing… something? Cram gets close but maybe not quite there.
Edit: I think it’s the fact that you get corrected upfront and that is going to affect your other answers and skew the results. I also think an evaluation would have different kinds of questions than an SRS - read a paragraph and select a statement about it, for example. That might be hard to execute on, but it would be really helpful to have more variety in the kinds of questions since they each exercise the brain in different ways.