http://forums.d3go.com/discussion/62286/current-available-characters-and-future-removal-additions/p1
Not quite up to date, but in chronological order at least. Next season should see Grocket and Gamora enter the fourstar token pool while Spider-woman and Luke Cage gets out. Star-Lord should enter the Three-star pool, but it is unclear who will be removed to make room for him.