How about adding the option of an auto repeat function? What I mean is that every time you get a "wrong", you have to write that word again until you get it right. I feel that this would greatly improve the rate of learning, since you are "forced" to get it correctly (punishment), since you won't be able to see the next word (reward) until you have gotten the first one right.
There is also a another level with this function, as you are in a way "punished" by having to tediously repeat the word. It might sound stupid, but it's actually psychologically proven that your brain will "want" to get that word right the next time. Small details like this actually makes a considerable difference for learning.
Maybe you could put it as an option on the settings page, and also add the option of choosing how many times you would have to repeat the said word, until you can continue. The word would still count as a "wrong" though.