This is just my opinion. But i really don't think Felicity is the one in the grave. Yes, she might die but there's a possibility that it might be someone else. For instance Malcolm Merlin has been making a lot of appearances. This constantly reminds us that Thea was resurrected from the dead, so there is a huge possibilty that it might be Thea. Maybe her bloodlust gets so out of control that she goes out on a silent killing spree. On doing so she might piss off the wrong people and thus gets killed. Or maybe Oliver is one who has to put her down, as this is said to be the only way to let the dead get peace. Also killing someone from the dead isn't something new. Even Lance wanted and was adviced to kill Sara.
(This is just my opinion and my first time writing.)