Here are some of my ideas that may be able to help Duolingo.
1) Buffer-zone: sometimes quickly switching languages can be tricky, especially if they have different grammatical structure. this buffer-zone asks ten quick, easy questions that could help you transition between languages. This could be disabled in the config menu.
2) Stacked streak freeze: This idea could help with vacation. You can stack streak freezes on top of each other. Each additional streak freeze costs two more lingots, to be fair. This would make vacationing with a big streak a lot less stressful.
3) Flagging: Duolingo could flag questions you made a mistake on, for one day. Therefore if you come across that question again, you would be encouraged to be more careful. This could be toggled in the config menu.
4) Timer potions: similar to the health potions from way back when, the timer potion costs five lingots and adds 20 secs emergency time for timed practice. another way to spend lingots.
5) Sentence sandbox: this feature would cost 10 lingots per language. It would basically allow you to write a sentence in your native language, then translate whatever sentence you wrote to the language you are learning. It could help with witting in different languages.
These are just my opinion on what I think would help learning a new language.
Regarding: 1) Buffer-zone:
You can do this on your own by watching a short youtube video or song music video in your next target language right before switching. This could be better than having Duo do it for you because it diversifies your experience by ensuring you get some non-Duo (not the same Duo - type sentences/format) input every practice session.
Another thing I like to do to help my brain stay in the right language mode is stream the news or music in the background in the language I'm studying at that moment (I turn the volume on medium-low so that the Duo audio is louder than my background noise of choice).
I think that these methods work better for me than a Duo transition would and I recommend trying them out to people struggling with the same thing.
I find rapidly transitioning to be a challenge when the languages are similar, not when they're drastically different. But these methods make it easy either way.
This is a great idea! I might try this when switching between Spanish and Turkish. Another possible buffer-zone would be Duolingo aromatically streaming a short radio story between languages. I can see how switching between languages that have similar structure could be even trickier than switching between ones that don't. The languages I happen to use the most have very different structures, so this could just be me.