Maybe you could have an advanced tree like for people who have mastered the basic tree they could move up and work on more advanced stuff or they could give you links to websites where you could get a deeper understanding of the language i.e: verb tenses, grammar rules.
I'd suggest to improve the existing lessons rather then adding new ones. Duolingo's method of repetition enhanced by gamelike experience is great for learning words, phrases and paying attention to subtleties like articles and endings. However, the choice of the phrases is often far from perfect. For example, in the French course Subjunctive unit there are many sentences that don't have anything to do with subjunctive mood but appeared there just because they had the word "que" that is also used in subjunctive. There is also a huge number of bizarre sentences that don't make any sense and don't help to remember words, like "ducks contain vegetables" or "leather is visible". If I were to decide, I'd prefer to polish the existing courses to make sure they really offer the most useful words and phrases and also to ensure that every phrase given corresponds to the unit topic.
It would be nice if people who fill trees could help preparing some more stuff, giving grammar hints and suggestions for new lessons, and similar. Usually people here are language freaks, and could give many suggestions on topics that are challenging.
Anyway I think the real world practice is designed to help learning even when you're done with the trees. :)