At the end of the tree, you should get a bunch of topical skills. These could be things like law, technology, etc. that help you with your fields of expertise, without becoming lessons you have to learn. They could cost as much as a bonus skill. Any comments?
Eventually as duolingo gets their most pressing work done to bring all the major languages on board, I expect to see more bonus materials and extensions of the early trees flowering. French from English already had a major upgrade not long ago. I'm looking forward to it!