Warning: Possible spoilers for the Arrow Season 4 premiere ahead. Enter at your own risk!
After the season premiere of Arrow, all any DC fan can talk about is whose grave Oliver and Barry visited at the end of the episode. While everyone will come up with their own different theories as the season progresses, I can't help but to speculate on who made the ultimate sacrifice. Is it a close friend? Is it an even closer "friend." Let's jump right in and try to figure it out!
There's obviously a lot of tension between Oliver and Diggle right now after what he did to John's family, but that's why I think he's the most likely candidate. If Diggle died while him and Ollie were at odds with one another, then he would never forgive himself. His death could be the result of him going out on his own because of his differences with Oliver, and that would be even worse.
Thea Queen (again)
When Team Arrow stormed the warehouse where HIVE was held up, Thea (or the Red Arrow) almost snapped and straight up stabbed a guy before Oliver stopped her. That's why I think that Thea is walking around with a target on her back. She's extremely damaged, and the show would get along much better without her there. How could she possibly die? I think that it could quite possibly be a member of Team Arrow having to put her down, or maybe Malcolm Merlyn decided to pull her plug because she refused to join him as a member of the League of Assassins. Or better yet, with Sarah Lance coming back from the dead in DC's Legends of Tomorrow, what if she decided to take revenge on her killer? It's a long shot, but it would make for such a refreshingly dramatic turn for the show.
Right now Constantine is only scheduled to be in one episode of Arrow as a guest star, but with the impending threat of Damien Darhk inevitability growing, he could very well be bumped up to a series regular. And if they didn't want to give him his own show back, or keep him on the show, what better way to say goodbye is there than to have HIVE off him. It would be sort of like an Agent Coulson situation. If their newfound teammate kicks the bucket, that could serve as the incentive for Green Arrow and the gang to take down HIVE for good, and could help us cope with saying goodbye to our favorite, cockney demon hunter.
Yes, it was revealed at the end of the episode that Captain Lance was working for HIVE, but he doesn't want to. It's either do what Darhk wants and keep him and his family safe, or let everyone he loves die, and that's why Lance hasn't gone to the "Darhk side" in my book. I think that Lance will defy Damien and the rest of HIVE, and have some sort of final moment with Oliver where they say some touching things and finally understand each other. That will be what pushes him over the edge. Not just the death, but the final moments, and what he died for.
Yeah, I know that you saw this one coming from a mile away, but bear with me. If Felicity died, finally making up her mind between Oliver, Barry and Ray, then Ollie would go completely over the edge. So much for the Green Arrow being a symbol of hope, because chances are he's going to kill everyone involved in her death. This would be a wonderful direction for the show to go, and get back to its darker roots that they may be straying from with Oliver going from The Arrow to Green Arrow. It would be sort of obvious to go after the love interest, especially after the reveal that Oliver was planning to propose, but it would still be the proper push needed to be able to take down HIVE.