SPOILER WARNING: This article contains major spoilers from "Blood Debts," the midseason premiere of "Arrow."
We have to talk about Donna Smoak. Donna has appeared in a total of six episodes of "Arrow" so far, and half of them have been this season. Prior to Season Four, she arrived to provide some comedic relief and occasionally offer Felicity solid motherly advice. Now, however, she's assuming a more critical role -- and the show may be aiming to build her up to ensure her death carries more of an emotional impact.
Donna's recent appearances have all aimed to forge stronger connections between her and the other characters on the show. When she showed up at Oliver and Felicity's loft, she gave them advice that ultimately strengthened their relationship. What's more, she's recently entered into a relationship with Captain Quentin Lance -- who has been secretly working with Damien Darhk.
If there's one thing we know about Damien Darhk, it's this: if you're on his blacklist, he doesn't come for you; he comes for the ones you love. Darhk makes a point of threatening the family members of those he forces to work with him. For instance, he has used Laurel against her father in order to keep Quentin in his pocket.
In "Blood Debts," Quentin made a difficult decision and betrayed Darhk by giving Oliver the location of H.I.V.E.'s base of operations so the Green Arrow could exact revenge for Felicity. When Darhk finds out -- and he will -- there's a good chance he'll go after the people Quentin loves, which includes Laurel and Donna.
If Donna dies at Darhk's hands, there's a good chance that would strain Felicity and Oliver's relationship. It would also explain why Oliver looks so taken aback by this turn of events; Donna isn't involved in their vigilante extracurriculars, so there's way less reason to suspect she'd get caught in the crossfire.
This would also explain Barry Allen's reaction to the death. Barry has interacted with just about every member of Team Arrow, and -- while he offers his sympathies -- he doesn't seem particularly distraught; he appears to be there only to offer Oliver some support. If it was someone he knew personally, he'd likely appear more affected. Since he has yet to encounter Donna Smoak (yet), this could explain why he didn't have a strong reaction.
Due to his aforementioned betrayal of Damien Darhk, Quentin Lance must also come under scrutiny. Darhk was deeply unsettled by the way Team Arrow discovered his wife and daughter, which may lead him to take a more direct approach with Quentin.
But why would Felicity be so upset by Quentin's death? Besides the fact they've worked closely together for years now, she may feel responsible. Oliver got reckless because she was injured; he pressured Quentin to give up the location of H.I.V.E., which puts Quentin and everyone he loves in danger. There's a very real chance she'd blame herself if something happened to him.
Additionally, there's no telling how close Donna and Quentin will grow over the next few months. Quentin's death could break Donna's heart, which would also ultimately affect Felicity.
Samantha and William Clayton
Then there's the issue of Samantha Clayton and her son William. In this season's "Arrow"/"Flash" crossover, Oliver discovered his ex-girlfriend Samantha had his child and that his mother had covered the whole thing up. When he confronted Samantha about it, she made him swear he wouldn't tell anyone -- even Felicity -- about their son, for fear of William getting tangled up in Oliver's messy life. So far he's been true to his word.
Before Barry went and erased a timeline to defeat Vandal Savage, Felicity discovered the truth and promptly ended her relationship with Oliver -- not because he had a child, but because he had planned on keeping it secret from her. Barry warned Oliver not to keep it from Felicity this time around, but Oliver didn't heed his warning.
As mentioned above, Darhk has a nasty habit of going after the loved ones of people on his blacklist. It wouldn't be out of character for him to target Samantha and William in order to get back at Oliver or the Green Arrow. Even so, child death might be a little too dark, even for "Arrow"; however, if Samantha is the one to die, then custody of William may fall to Oliver.
If Samantha dies and Oliver is suddenly in charge of his son, it'll likely come as a nasty shock to Felicity -- which would explain her frosty reception of Oliver in the flash forward. What's more, Barry is the only one besides Oliver to know the truth, which would explain why he came to the funeral to lend his support.
Laurel Lance is in danger the same way Donna is: through her connection to Quentin. However, Laurel has a higher risk due to the nature of her affiliation with Team Arrow. She voluntarily puts herself in danger by fighting H.I.V.E. alongside Green Arrow. If Darhk were to learn she was Black Canary in addition to Quentin's daughter, that would paint an even bigger target on her back -- or she may simply get caught in the crossfire.
Just because we focused on a few characters doesn't mean anyone not mentioned is safe. For instance, Diggle is like a rock for the team; if he were to die, it'd be a tragedy not only for his team, but for his family. Thea continues to have a blood lust that could push her into recklessness. Roy is returning to Star City. However, it seems less likely they will be the ones to join the choir invisible.
Of course, while much of this discussion hinges on Damien Darhk, it's worth pointing out that Darhk has never been named during the flash forward. Oliver and Felicity both only refer to the antagonist through pronouns: he/him. While it seems likely Darhk is indeed the culprit, it's worth noting that Shadowspire is headed to Star City soon, Andy Diggle hasn't come around to Team Arrow's side quite yet and Malcolm Merlyn still has dubious motives (especially after resurrecting Vandal Savage). However, as Darhk is truly the Big Bad of the season, the odds are slightly more in his favor here.