App Immersion !
I really think Duolingo should add the immersion section to it's app. I know maybe the size of it would become a bit bigger, but it is essential for people that have ended the skill tree and want to improve their abilities without needing to use the website.
This is something we're thinking about over at Duolingo :) Finding the best way to do this!
Perhaps a separate app which will make it easier to implement and it will not bloat the learning app.