New Season 6 set images may be hinting at the Negative Flash as pics from Vancouver apparently show a much darker Barry Allen.
In the comics, Barry Allen becomes the Negative Flash when he is trapped inside the Negative Speed Force by the Reverse Flash, who was responsible for its creation when leeching off Barry's powers throughout history.
Last Season on the TV series saw Barry's daughter, Nora West-Allen, taught how to access the Negative Speed Force in order to travel back in time so her father wouldn't notice, which eventually led to her death and the survival of the Reverse Flash.
It's possible that Barry could become the Negative Flash in the show while trying to undo his daughter's death or possibly having something to do with some sort of battle with the Reverse Flash.
Regarding the Negative Speed Force, according to Eobard Thawne, it is the only place in existence that is immune to timeline changes, so a Speedster can survive inside the Negative Speed Force if their timeline gets altered or destroyed. The only problem is the Speedster is dependent on the Negative Speed Force for its speed (negative energy) unless the Speedster's timeline is reinstated (Speed Force - positive energy).
Negative Flash pics?
The Flash Season 6 premiers Tuesday, Oct. 8 at 8/7c starring Grant Gustin, Danielle Panabaker, Carlos Valdes, Tom Cavanagh, Jesse L. Martin, Danielle Nicolet, and Hartley Sawyer.