I had no idea it happened here, but as is common place, especially for older shows, some series regulars aren't signed for whole seasons. Thea hasn't been around because Willa is only contracted for 14 episodes this year. I'd guess Paul Blackthorne too. Beforehand I think it was just Barrowman, but I guess now that money is going to paying all the new recruits. Thought Willa was shooting a movie or something. And they want Katie Cassidy back for a stretch but it seems her series regular contract or whatever it was doesn't guarantee episodes so she hasn't signed on for the arc. Pilot season may snatch some of the non contracted actors. #CancelTrainingDay
http://tvline.com/2017/02/13/arrow-season-5-katie-cassidy-contract-status-willa-holland/
Also there was a press screening of this week's episode which can potentially mean something major.