Arrow: 6 Ups And 5 Downs From The Season 4 Premiere 'Green Arrow'

Arrow's done plenty with flashbacks, but right at the end of the episode we get a flashforward, and it's a pretty huge one. Jumping six months into the future, it shows Oliver and Barry at a gravestone following a funeral (confirming this speculation).

It confirms a pretty major death, presumably at the hands of Damien Darhk (and there's also a mention of the trouble Zoom is causing Barry), although it doesn't actually reveal just who the grave belongs to. However, it has to be someone close to Oliver and at least a little close to Barry. 

Right now I'd say that the most likely candidate is Detective Lance, given he's working with Darhk but likely to try and get out of this situation. Outside of that it's a lot of speculation and could be any number of characters, but it'd be a huge shock if the likes of Thea or Laurel were to be killed off at this stage (though of course that may be part of the reason to do it), and I'm not even entertaining the idea of it being Felicity. Just, no.