Will Progress tests come back and I think a good idea would be a grammar section for each language.
I agree. Grammar notes are sorely needed across the board in DL. They are almost non-existent beyond the first checkpoint in Spanish, while many new concepts are introduced thereafter. Many other users have complained of the non-existence or a lack of notes in their courses too. Since DL teaches us more grammar than other areas of language learning it seems sensible that support notes be included, and it always baffles me that there are not more. I came too late for progress tests but they sound like a great idea - assuming they are developed properly of course
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.