Word of the Day/Week
I feel like I keep seeing the same, easy words I already know over and over mixed with the occasional, unfamiliar word I see once and then never see again. Repetition is important for retention.
I think it would be a neat feature to have a Word of the Day or Week where every day/week, there's a new word (preferably less-common) that any time it appears on your screen, it'd be translated (like your vocab words but temporary). I, personally, would like this repetition to learn new words on a regular basis.