1.) It will have serious impact- That is the main thing that the writers are looking for. Whoever is in that grave has to be close enough to Oliver to bring him to kill. We have already seen that he is willing to kill for her when he took out the Original Count Vertigo in Season 2. The same could said for Thea, but would not have the same impact as killing off one of the original members of Team Arrow and the current love of Oliver's life.
2.) Barry showed up- When Barry apologized for missing the funeral, Oliver naturally assumes that the reason Barry was late is Zoom, which Barry confirms. That means whoever is in the grave must not only be important to Oliver, but also be close enough to Team Flash that Barry comes running to make the funeral after beating his big bad. Felicity is always popping up in Central City and it's that clear she and Barry share a special bond. While Barry is the kind of friend who would come to console Oliver regardless of who died, Felicity is the only one who could make him come so quickly.
3.) They don't really need her anymore- The main role of Felicity Smoak on Team Arrow besides the IT genius is the light and moral compass. With people like Oliver and Diggle who have seen war and the darkest parts of humanity, it is important to have a person who sees the world in black and white, to keep those who live in the grey from becoming like Slade Wilson and Malcolm Merlyn, men who have become the very things they once sought to destroy. Or worse. With lighter-hearted heroes like the Atom and Mr. Terrific moving into Star City, Oliver has new allies to keep him from giving to his dark side. Since Ray also loved Felicity and Curtis Holt is being sort of mentored by her, it stands to reason that they will definitely be there to make sure Oliver does not forget the lessons that Felicity strived to teach him about embracing his light.
4.) The return of Arsenal- Roy and Felicity have always shared a special bond. When Roy first thought that he might still have Mirakuru left in his system, it was Felicity was the one he confided in. When Sara died, Roy was always the first comfort here whenever she broke down. When Roy left Starling City after faking his death, Felicity told him to expect plenty of calls on his new SAT phone. And she knew where he was currently hiding out when Thea wanted to start a life with him. While Felicity's story seems to be coming to an end, Roy's story seems unfinished, having left the show after only half a season as hero when he spent the past two seasons searching for a way to join Oliver's crusade. Colton Haynes, the actor who plays Roy, has already confirmed that he will reaper for a few episodes in season four. Felicity's death could give Roy the motivation to put back on the hood and return as a regular in the next season. The same could be said for Thea, but then chances are it would be Roy meeting Oliver at the grave instead Barry.
5.) Green Arrow and Black Canary- (Sorry Olicity) This is a major relationship in the DC comics. I'm sure that plenty die hard Green Arrow comic fans got excited when they learned that Laurel's full name was Dinah Laurel Lance. In the first season there were plenty of sparks flying between these two but since the death of Tommy, it seems that the fire has died. That is because when it comes to major DC comic love stories, the CW loves to make us wait. In the Superman TV series Smallville, Lois Lane was introduced back in season 4, but sparks between her and Clark Kent didn't even really start to fly until season 8. Oliver and Laurel has always shared a special bond. When Sara died, Oliver was always the first to comfort her. When Oliver was ready to hand himself over to Slade, it was Laurel was who convinced him to keep fighting. When Oliver was on the island with Slade, the first person he tried to reach was Laurel. When Laurel first saw Oliver's scars, Oliver admitted that the one thing he wanted more than to die and let the pain end was Laurel. Oliver himself even admitted to having a blind spot when it comes to her. Felicity's death will give Laurel the opportunity to be there Oliver the way he was there for her when she lost Tommy and Sara. The reason that the writers did not focus on this right away was because Laurel had to go through her own journey and come into her own as a hero before the writers could focus on her love life. But the bond between these has always been there. Now it's time to rekindle the flame.