Association within languages
It would be useful to add another type of exercise, where words appear in pairs, but all words are in the language the user is learning and the goal is to pair up words with similar or identical meanings (warm-hot, grandmother-grandma, hi-hello).
A variation to this would be pairing words that are each others opposites (hot-cold, fast-slow) or pairs where one word describes a category and the other an object or concept in that category (apple-fruit, running-sport, sad-emotion,green-color).
That would be a good idea especially for words with more than one meaning. It would expand vocabulary, not just by adding in new words but it could also teach new ways to use words already learned.
I've had experiences to a similar effect with the matching activities on the mobile app in the Portuguese for Spanish speakers course (neither of which is my first language). I first started this course in addition to the Portuguese and Spanish for English speakers courses for the sole purpose of better differentiating between the two since they're very much alike. However, I've encountered situations in the matching exercises in which I knew one English translation each for the Spanish and Portuguese words, and based solely on those, the words didn't match up. This showed me that the words could be used in more ways than what the English speakers courses taught me.
I like your idea. I feel like one downside of Duolingo as it is now is that it doesn't teach a lot of synonyms so once you know one translation between your native and your target, you stick with that usage instead of adding variety. Association exercises could improve that.
There are Portuguese and Spanish test sites which test for synoyms and antonyms.
You have to decide which words of a 4-5 word list is the correct answer.
I find that quite challenging.
The same occurs for "filling in words into blanks", where you have to e.g choose the grammatical correct version and you only see in () the base infinitive form of the word, or even only the English word.
I tried a test where there have been even TWO blanks (beginning and ending or 2nd part of the sentence after ",") in a longer sentence.
That also was quite a challenge for me.
I somehow miss the Android app "word pair" excercise on the website, which get's thrown into between doing lessons. Nice. This nicely refreshes old vocabulary in the easy way.