User blog comment:TheDoctor230/Flash Season Final Theory (Spoilers)/@comment-3221148-20160513110715

Tom Cavanagh also said that there is some special scene that he's been "hiding" until the finale that would bring all the plot points related to Harry Wells throughout 23 episodes to an epic conclusion. This made me think.

The plot points connected with Harry: his hatred of Zoom (he distrusted him back as Jay Garrick, and then promised to "knock that smug expression from his face" personally), his unexplained knowledge of speedforce and time travel (he IMMEDIATELY recognized that the "vortex" Cisco vibed Barry in was the speedforce, he explained what a speed mirage and a time remnant is, something that a physicist shouldn't know, finally, he has a Time Vault, something Thawne added secretly, and even calls it the Time Vault), his love for Jesse (note that when she asked him to promise not to kill he only promised to be "a father she'd be proud of"), his acquisition of powerful devices (the serum created from the Turtle's ability, the speed-enhancing potions, Eobard Thawne's suit, notes and tachyons).

Whatever big twist would involve Harry that would be related to his activities throughout the 23 episodes of the season, it surely involves: the death of Zoom, the speedforce, Barry being the Flash, and Turtle's ability.

I think Harry's betrayal will lie in Barry's refusal to outright kill Zoom. Maybe Caitlin finds out something about Hunter that will make her convince Barry that he can still be saved. Harry, who wants revenge on Hunter, will be angry at them, and use the combination of speedforce and Turtle's time conduit ability to get superpowers and kill Zoom on his own. Maybe he'll become a meta-human with an alter-ego of his own (Professor Zoom? Johny Quick? The Rival?)

On Caitlin: I don't think she'll just get Stockholm Syndrome and join Hunter outright. However, she will try to sympathize with him and ask Barry to spare Hunter from death. I also think she will, eventually, become Killer Frost after all the emotional stress and abuse from her inability to protect herself she has suffered, but as an anti-hero of sorts, rather than the campy villain that Caitlin-2 was. And she'll stay brunette!