Resources after finishing a tree
I recently finished the entire tree in three languages German, French and Spanish. What I noticed is that, once we complete the tree, there is really no way to gain a lot fo XPs in that language. We can only gain as much as one revision test. In contrast, if we are starting off in a language, we can test out of a skills in one go and gain a lot of XPs (like, if there are ten lessons in a skill and we test out of it, we get 100 XPs directly). This really makes it really hard to compete with peers in leaderboards in groups. While those who are beginning the journey in a language are able to quickly get XPs, those who have completed the tree are unable to scale up and reaceh #1 spot on the leaderboard.
Duolingo should give some serious thought into fixing this anomaly. Duolingo's main selling point as I see it is how we can learn a language, while being in a play mode. If finishing a tre really retards competition, then the incentive to further practise that language will diminish.
Why not do German-from-French, French-from-Spanish or any other combination of these three? It's a great way to look at either language from a different perspective, while also gaining a lot of exp from testing out different skills. If that sounds too boring, you could also try the Italian tree from Spanish or French (an Italian-from-German course sadly doesn't exist), or Portuguese or Catalan if you really want to try something new, while still learning a bunch of new stuff about your "original" target languages!