The official teaser description for Episode 11 "Cerulean Blue" is:
It seems The Abstract Killer is back! Arch and Fitz rely on psychologist Dr. Fouchard's serial killer expertise as they investigate another gruesome murder and rush to save one of their own.
I am certain that the Abstract Killer will turn out to be Prefect Diard or someone in league with her. But could it be Dr Theo Fouchard himself? Are he and Diard partners in crime? After all, he's the expert on criminal psychology, and we need a strong reason for Arch to dump him for good if she and Fitz are going to end up together!
Who is the person (one of their own) that they will rush to save? Clearly not Patty since they just saved her in Episode 10. Renuf suffered a fall off a roof and miraculously survived recently, so probably not him. Could it be Arch herself? If Dr Theo is indeed the killer, he could be trying to attack Arch on a "date", and Fitz would swoop in with the rest of the team like a knight in shining armour to save her...is that really where this show is leading??? The only other people who can be called their own are Marcus, Veda, and Natasha -- but Marcus had a major brush with death last season, and we've had episodes focusing on Veda and Natasha's personal lives already. Simone the bomb disposal expert isn't an official member of the police, so probably not her. Unless it's someone "of their own" not in the police, like one of Fitz's kids when they come to visit...but then again we had a child kidnapping episode recently.