It's pretty amazing the number of fairly major characters who have not been seen in a while: Littlefinger, Varys, Margery, Loras, Olenna, Brienne, Tommen, Gregor Clegane, etc.
We've seen most of these characters this season, but but in a few weeks.
Prediction for next season: New actor will play Tommen. The king is supposed to be a weak and clueless boy. The actor who has been playing Tommen will be 18 next year. Very hard to pull off the "I'm just a child who isn't ready to be king" bit if you're 18.
We haven't seen Tommen since (I think) episode 5. The actor is forgotten and easily replaced.