Suggestion: Automatically build a grammar section that is just like the vocabulary section but restricted to those words that learners typically have difficulty with (as determined from the data that Duolingo has collected).
And of course you would be able to practice your weakest words in the grammar section so that you could focus on improving your grammar.
Oh! I was against having Grammar here in DL, because at least I think, that it's just not the same, it makes it harder, I prefer DL without grammar, besides the community helps. But with your idea, I think it would be ok. I like a lot the idea!!!! And that way, I can enter when I want to practice or when I have a doubt about grammar, and not having it interfering with my lessons.