Maybe I'm wrong, but it sure feels like this game repeats the types of traitstones available for purchase on a fairly regular basis. This week, we're back to Arcane Forest. I'll still buy some to get closer to traiting out Ketras, although I need some Arcane Beast and Celestial stones as well.
Other useful troops using Arcane Forest:
Famine: if I have to describe the usefulness of Famine, you probably haven't played the game much ...
Yasmine's Chosen: I don't have this troop and haven't seen it in action much, but it looks pretty solid
Sylvanimora: One of my favorite dragon troops, it's a solid exploder and the third trait (Dragon Vines) entangles enemies, making skull-baiting a possibility
Soothsayer: Good low-cost troop to fit into green or brown teams--when fully traited, it starts with full mana, so it allows a solid turn 1 start.
I'm sure there are others, but I don't have or use them ...