Who is going to die on Arrow? Someone is going to die. We know it's not Oliver or Felicity, and it's probably not anyone from the Flash. So far, I have five theories: it's Diggle, William's mother, Lyla, William himself or Felicity's mother.
I know it stands as a kind of leap or stretch, but these are people whose deaths would leave emotional reactions from both Oliver and Felicity. So basically, these characters are being scaled comparatively between Olicity. The person's death has to have a severe reaction from the both of them.
1. John Diggle
Diggle is Oliver's best friend. He's also Felicity's close friend. His death would probably be the one to cause the most negative reaction. Diggle has been with the show from the very beginning and, based on the reactions, I have accepted the possibility that the person going to die may very well be someone already well established in the show. Diggle's death would provide quite a shock and would be the perfect cliffhanger to end the season.
2. Samantha Clayton
This character, although not a major one, would provide major conflicts for the other characters because without his mother, William would be left in the care of Oliver or a possible other that has not been introduced yet. However, Felicity would most likely be unaffected by Samantha's death and therefore it is unlikely she will die.
3. William Clayton
So if Samantha is not going to die, the next logical person of interest would be William. His death would destroy Oliver, and even though Felicity has forgotten his existence, via Barry's time traveling, she could still find out later. The death of an innocent child would also affect her as well, especially Oliver's child. This would also mean that the person Felicity tells Oliver to kill would be Merlyn. Thanks to Malcolm, Damien Darhk knows of William's existence, so even if Darhk does cause William's death, Merlyn could still be held accountable.
4. Lyla Michaels
Lyla is also a close friend to Oliver and Felicity. If she were to die, Diggle could go into a violent, downward spin. This could also result in further conflicts that wouldn't come into play until the next season. However, Felicity may not have that major of a reaction, so her death is also unlikely.
5. Donna Smoak
Donna is not a major character, but she is slowly becoming a fan favorite. Her presence brings a certain comedic touch to the show that is lacking in other characters besides Felicity. Her death would most definitely devastate Felicity. Oliver's reaction in the limo with Felicity is sombre. This suggests that he is grieving, but he does not seem to be the person grieving the most. That person is obviously Felicity. This makes her death even more likely because future Felicity was absolutely destroyed by the death after the funeral.
Basically someone is going to die, and we don't know who. I believe that it will be one of these 5 aforementioned souls: Diggle, Lyla, William, Samantha or Donna. Now, if you were to ask me which one would die, I would most likely tell you it's Donna because of Felicity's extreme reaction. If we are also to follow this thought process it could suggest that the "he" that Felicity refers to is not Merlyn, as previously suggested, but actually her own father, Noah Kuttler/The Calculator. Kuttler was turned into the police by Felicity in the latest episode. This could provide the motivation he would need to seek revenge on Felicity, which he would carry out by killing Donna.