1. ATLA-inspired hunt. I like the idea above where you can use which element to bend and it corresponds to an appropriate avi. The leaderboard awards could include like a nation's leader (like for those who choose fire, their awarded male avi would look like Ozai). And the top ones in the winning nation or the one with the most drops would get an Avatar-inspired avi, male one could be inspired by Aang, female is like Korra.
2. Campus Elections hunt! Maybe three of the characters would run for student council. You could pick a candidate to support. Then you get campaign banners as drops, which will turn into voting ballots, and then you could hit a certain player (like "PollWatcher") to turn in the votes for the candidate you support.