Will Progress tests come back and I think a good idea would be a grammar section for each language.
You can get the hidden "progress quiz" back for yourself: https://www.google.de/search?q=site%3Aduolingo.com+duo.old_web_url_whitelist%3D%5B%22%5E.%2B%22%5D%3B
I think Progress tests are gone for good. But who knows, Duo never ceases to amaze me with their actions. I always thought the tests were useful but a lot of users complained because they did not give any feedback.
I disagree about you point of a grammar section per language. The tips and notes section in each skill already do that and allows you to study grammar in small chunks relative to the subject under study.