See your errors after practicing
I think it'd be good to be able to see all the mistakes you've made after you've finished your exercise, especially with timed practice, since you don't really have the time to look and understand what you did wrong.
I really love the proposal! Maybe it is even possible to give a short summery on a higher level. E.g. pay attention that a certain noun has a different gender, or don't forget gramatical construct xx. In a later duolingo version, this could be extended across different exercises and give further feedback on personal weak points.