I have my doubts about Diaz, but Caden James...I mean, we went from Prometheus, who was a badass super-villain to some regular guy? He's dead now but now it's Diaz, and well I'm not convinced as of yet. I think it was a bad idea to kill Malcolm off. I liked him being a recurring character. He was like Lex Luthor was to Superman.
As they have already wrapped filming, I doubt we'll be seeing Deathstroke return before this season wraps.
I hope season 7 goes into a completely different direction altogether. Maybe Oliver should stop being Mayor.
I agree though, the season-long arcs aren't doing the show that much good. Notice how Gotham hasn't done season-long arcs. They usually do two story arcs per season.