Mems to remember the words
On memrise.com there's a very useful feature for memorizing words. Adding mems. It could be some word, sentence in mother language, picture or anything else that helps to associate the new word. I find using mems really helpful. In fact, I believe it to be the best technique for memorizing new words. It also would be nice to have vote ups for the mem you like but that's optional.
So my suggestion is to add this feature to Duolingo!
I'd rather leave that to Memrise since they're doing it well already, and see Duolingo expand in other ways. But I highly recommend using both at the same time.