Will we see League of Assassins members like Ra's, Talia, Lady Shiva, and Cassandra Cain? But I guess the villain of the season is Brother Blood. So they'll most likely focus on him.
Unlike Smallville, there doesn't seem to be any supernatural or out-of-this-world element on this show that would include superpowered beings, aliens, magic folk, etc. Otherwise, they would've shown a different version of Blood, or a Black Canary with a real sonic scream.
Cassandra Cain in the LoA? Really? Her "evil" phase was almost universally hated and even DC, who loves to stick by their boneheaded decisions, back peddled furiously from it and explained it away as Cain being drugged after the fans completely freaked out on them. She's really not someone connected to LoA outside of that almost universally hated story arc, at least not as a member. Her mom, Lady Shiva, was a member though. I don't think we'll ever see her in live action, but if we do I don't think even Arrow, with its lack of caring about the comics, would use an evil, assassin version of CC. I could see Lady Shiva, talia and other better known evil characters, but not CC. It would be like introducing Nathanial Adam (aka Captain Atom) but have him show up as Monarch or having Dick Grayson show up as Renegade.
To be clear, I'm a huge fan of Cassandra. She's in my top 5 favorite DC characters. I just hope we never see her in Arrow, at least not like that. I could see her working in the show if done right, though. Introduce Shiva, then bring in Cassandra as someone hunting Shiva/the LoA, and it could work. You could even make it a BC/Lady Shiva/CC thing. But as much as I love Arrow, evil CC would be something that would make me stop watching. I'm just glad there is basically no chance of seeing her or someone with her name, because I doubt they'd use her right.