One thing I liked about DuoLingo is they would introduce words in content sections (the home, jobs, education, etc). However the sucky part of duo is there's not enough words to get you past A2-ish which is why I'm loving Lingvist after having discovered it a few weeks ago.
It would be amazing (and marketable) if there were word modules you could access that were for specific topic areas (business, education, healthcare, teaching, etc). Since those are professional areas, charging professionals who want to access them is a very reasonable (and much less dickish than duo's) way of monetizing content. OK, admittedly it's kind of like a drug dealer... the first 5000 words are free... But it would be amazing and make Lingvist more useful than duo in yet another way (than just being what one graduates to).
Personally if there was a medical module I would readily pay $200 for access. That might be steep for a home health worker or an RN, but it would be a bargain for me.