In the season finale of The Flash, after Eddie killed himself to wipe Eobard (AKA Harrison Wells, or the reverse flash whatsoever) from existence, we've seen a giant black hole-y thing popped up in the sky and started to suck everything into it. That is much likely to be indicating that history is mending itself (like from Doctor Who) since this entire "Flarrow universe" would never have existed if Eobard never travelled back in time to kill Barry's mom (Or barry) nor if he never existed.
And here comes the main thing. We all know that the flash and the arrow exist in the same universe. And we've seen Sara Lance AKA the canary resurrected by the lazarus pit. But hey, isn't only Ra's Al Gu the only one allowed to use the pit? What if, the entire existence was all flipped upside-down just because of the reverse flash's death? That would make perfect sense though, plus, Tom Cavanagh (Harrison Wells/Eobard Thawne/Reverse flash) is still in the crew for season 2 and Arrow's chosen a new costume and renamed as the green arrow? We initially thought that Sara was simply brought back to life by Nyssa or someone else, but what if they're telling you that in that "universe", Sara is the Ra's? Well, the entire parallel universe(Or is it the original universe? I dunno) thing would explain A LOT about all these changes!
Oh I forgot something, while barry was in the speedforce rushing his way into the past, he saw Caitlin as the Killerfrost, which is much likely to be a glimpse into a parallel universe, and that adds one more point to the theory I guess.