Expanded, customizable practice area
I think it would be greatly beneficial to have an expanded practice area that allows you to customize the way you practice past lessons. For example, if you wanted to focus on verb conjugations or vocabulary or speaking or whatever, you could choose those specific things. Also, I would like to be able to choose the types of questions while practicing past lessons. For example, I feel direct translations are the most challenging and thus the best way to learn (as opposed to, say, multiple choice) and I would like to be able to choose to do only those during practice.
As it is, when you practice past lessons you get the randomized questions that might not cover what you most need to, and you get the mix of types of questions, like multiple choice and spoken reading, when you really want to, say, hammer away on some vocabulary translations to really get them down. There are plenty of things from past lessons that I haven't mastered, yet because of the current set-up I have to answer many simple things I already know while only occasionally being challenged with a concept or word I have trouble with. As a result I feel like I can't master past lessons as quickly as I'd like and, while wading through these randomized mix of questions, am learning at a much slower pace.