I boldly predict that Thea Queen is in the grave. I think we will get a curveball at some point where Thea dies. Her separate storyline is dragging and to me it's setting up tension between Malcolm and Oliver, which is why you see Oliver say "I'm going to kill him" out of anger as opposed to sadness. At this point in the series Oliver has completely lost his baby sister, she isn't blood thirsty right now but I believe Malcolm will do something either to her or something that will affect her to the point where Oliver will only seek vengeance.
Colton Haynes will be reprising his role as Roy Harper as some point on the show and according to his IMD'b page he's signed on for a quite a bit more episodes so perhaps he will become the Arsenal again?
I'm sticking to my original thought, which is Thea will die. Felicity Smoak obviously would be the worst character write off in resent memory (on shows I watch) at this point it wouldn't make much sense. I get teasing it from a ratings standpoint but if you just sacrifice another character without any sort of proper send off or quote on quote "farewell episode" then you'll end up with upset loyal fans. Basically what you'll have is the problem the Walking Dead had when they killed off Beth(if that's a spoiler for you than shame on you for being so behind), it came out of nowhere and essentially made no sense from a story perspective.
Thea is the only other character on Arrow that is getting what I look at as an exit scenario. So I'm going to stick with Thea Queen.
This may be too dark, but what if the person in the grave is Olivers son? Or at the very least his kids mother? Perhaps Damien meeting with his own family at the close of the first half of the season was a sick but albeit brilliant anomaly?