Barry and Iris's daughter travelling back in time is a huge deal, but Caitlin's Killer Frost storyline is something that I'm personally really excited to see unfold. In season four, we find out that Killer Frost has always been inside of Caitlin. Cisco vibes her to a past event in which a young Caitlin falls off her bike and almost gets hit by a car, which causes her to turn into Killer Frost. When the young Caitlin is lying in the street and is about to look at her reflection in her bike's mirror, her dad yells, "Caity, honey, don't!" Many pointed out how her father didn't seem surprised or stunned at his own daughter turning into a completely different being. Instead, he tries to keep her from seeing herself.
There are multiple reasons why he might have had that reaction. Maybe he's just good under pressure and he didn't want Caitlin to be scared. A more intriguing explanation is that Caitlin's father was a metahuman and knew that he passed the superhuman gene down to her. Caitlin has previously said that when her father was diagnosed with ALS, he hid a lot of his symptoms from the family. Perhaps it's because they were affecting his powers. Maybe Caitlin has always been Killer Frost, but the particle accelerator reawakened the icy alter ego. That also brings up another theory that's been floating around: it's possible that metahumans existed before the particle accelerator explosion and the event simply triggered and strengthened the powers they already had. That would totally flip the script on our heroes.
Evidently, there is so much that awaits us in the new season, and we're hoping that the show gets right into the action because WE NEED ANSWERS.