Idea: Motivate learners to complete lessons using an "addventure" where learners can add a sentence making use of at least one word from the completed lesson.
Another requirement would be that all words in the sentence must come from the just completed lesson or previous lessons.
There would be only one addventure for each language. You would want to complete lessons to see what happens next and to practice the language you are learning by contributing to the nonlinear story.
"An addventure, also known as a collaborative gamebook, is a type of online interactive fiction that combines aspects of round-robin stories and Choose Your Own Adventure-style tales. Like a round-robin story, an addventure is a form of collaborative fiction in which many authors contribute to a story, each writing discrete segments. However, like a gamebook, the resulting narrative is non-linear, allowing authors to branch out in different directions after each segment of the story. The result is a continually growing work of hypertext fiction."