About the Vocabulary Feature
I think it's a good initiative.
Ways it can be Improved:
I hope it can be extended to web versions too. One thing that'd help is audio, to have different native or equivalent speakers pronounce the words so we get a good mix. Another thing is finding different sources where a particular word or phrase is used, e.g. in movies, books, shows, social media, etc.
Gamification would also attract a lot of people. I find it quite nice to use as well. For example, giving points for every word revised or for every correct answer. From there, determine the strength of that word using bars, perhaps 5 bars like we have now for our proficiency in a particular language (A1, A2, B1, B2, C1 and C2).
One final thing, perhaps the Vocabulary feature could be used in line with Classroom, where we can add words we learn in Lessons automatically (or easily) and it'll allow us to review those words. Audio, sources and gamification can then apply. It'll be a whole system! A useful one, too.
What do you think? :)