I would like to see a lot more bonus skills in ALL languages
There is so much that can be done. Instead of waiting for Christmas, there could be a bonus skill for all of the holidays, profanity, sex, religion, mythology (if you're inclined to separate them), astronomy, archeology, astrology and the occult, science fiction, archaicisms. Even crime and punishment.
And as a lifelong learner and an English speaker I would even like to see an expansion specifically for fluent speakers that would focus on uncommonly used words and expressions, increasing the likelihood that everyone on Duolingo become Shakespeare's, Victor Hugo's and Sir Walter Scott's.
I already have nothing to spend Lingots on and I'm only at 11% fluency in French.
Please, feed my Faustian urges.
Bonus skills have the nice property of being optional. Skills like those mentioned by the OP are useful only in certain circumstances, and some people will never have an opportunity or inclination to find themselves in those circumstances. This way, only people who actually care about those skills will learn them, and they will most likely be at an appropriate skill level thanks to all the previous lessons they had to take to earn all those lingots.
Of course there should be some balance: a tree with 20 normal skills and 20 bonus skills would be awful for everyone involved: you'd have 40 lingots for skills, 44 for reaching level 8, and maybe 10 more for a 40-day streak. 94 in total, enough to buy 3 bonus skills and nothing left for progress quizzes, streak freezes, or the remaining 17 bonus skills.
Thanks for this great explanation. And I would have made Idioms a bonus skill too. Since I've been learning French I've observed that I'm more often converting converting idioms from one language's idiom into another. Often the words are nothing like each other, only the idea presented is identical.
Also if in the unlikely chance all of my suggestion were incorporated, there would still be only 1 bonus skill for every 5 normal ones.