On The Flash, Barry Allen uses his speed to take down villains, but it also requires a lot of cunning and brain power. It takes neither cunning nor brain power to surmise what the title of Kevin Smith's second episode of The Flash is, despite the director's best-ish efforts to keep it from people's eyes. If you take a good look at the tweet below, you'll see that the episode is named after Killer Frost, that most cold-hearted of villains who was killed off in Season 2.
While that pen definitely covered the "Killer Frost" title on the main page in that tweet, Kevin Smith couldn't stop paper from being transparent, and you can pick out the Earth-2 villain's name on the top of the next page. Perhaps Smith did it on purpose this way, since he knows that getting fans excited is one of his best attributes. And it's hard to think of something more exciting than getting to reconnect with Killer Frost, who was rather unfairly taken out of the narrative by Zoom.
So, as it goes with just about every character on The Flash, the discussion turns to "how in the hell is this actually going to happen?" Killer Frost was already the Earth-2 version of Caitlin, so that option is out, unless the story goes back into the past on Earth-2 to take on a different part of her story. Over on Earth-1, the season will start out with the limited Flashpoint arc, so it's possible that timeline shift could have had ramifications on Earth-2, making it so that Zoom didn't need to kill Frost. Or maybe she's from Earth-3 or beyond. Or maybe "Killer Frost" is actually referring to a freak weather phenomenon that affects Central City.
When Caitlin and Killer Frost got to share words last season, the topic of family came up, with both women's respective mother and brother situations getting broached. We're going to finally meet Caitlin's mom in Season 3, with Susan Walters taking on the role, so it's entirely possible she might have something to do with Killer Frost's return. (And it better be more than that faux reprisal when it was just Caitlin in cosplay.)
Given "Killer Frost" will be the seventh episode of the third season, we'll get to see what Kevin Smith has in store for his sophomore Flash effort at some point in November. Now we wait to hear about where Jason Mewes will show up in the episode.