Bonus grammar skills
I had a thought, looking around trying to learn french I learnt that there are some tenses within french that aren't really used anymore but still pop up from time to time, especially in literature. I understand that duolingo doesn't really try to teach us absolutely everything but instead gives us a strong foundation to be able to use the language and learn on our own. So these parts of the language might not fit in anywhere in the tree but I think it would be great to have stuff like passé simple, futur anterieur, etc. as optional skills.
I believe it will happen sooner or later but I can't really see how the more literary tenses would fit in the tree. Knowing them isn't a must but it helps especially if you intend to read a lot. Therefore I believe they would much better fit as optional bonus skills than mandatory skills in the tree.