For every fan desperately hoping it isn't Felicity, there's probably one hoping it is. She's a fairly divisive character, depending on whether you find her personality charming or annoying, enjoy Olicity or hate it, and want the show to follow the comics and have Oliver get together with Laurel. Based on what we saw in that brief scene, this one would fit from Oliver's point of view. He'd definitely be crying, be the last one remaining at the grave, and so angry that it would drive him to wanting to kill the person responsible. On the other hand, Barry and Felicity are pretty close, and he didn't look too upset. Furthermore, Felicity is a huge character, one of the original members of Team Arrow, and killing her off just as her and Oliver have finally gotten together seems like an unnecessarily brutal step that I can't see the showrunners taking. I hope not, anyway.