Once Upon a Time
How about incorporating a story into the lessons?
After finishing the first level in French, I realized that a story could make the experience quite more memorable.
The current process repeats mundane and occasional cute phrases. However, if there was some sort of a narrative that emerged, a quest, then this could make for an exceptional experience. These stories could also have social and historical context, and that would make them even more powerful.
Duolingo features an owl, why not take it a step further. Run with the connotations, give us a heralding figure to guide our quest.
I can foresee at least two problems like this the first and most important of which is that it would take time and a lot of research if you want to go include social and historical aspects and secondly lots of people like different types of stories meaning that a large amount of people would want to have different story options (more work for staff) and another group would think it childish and want everything to go back to the way it was.