Firestorm.
The last episode of The Flash where Ronnie and Stein go off to Pittsburgh shows that they've developed the characterization for them AND they are interesting.
While I wouldn't at this point believe the version of Caitlin Snow we've seen on FLASH becomes the villain they could have it be the original version, Crystal Frost as Killer Frost.
Let's face it, Caitlin is going to be a supporting player on The Flash for the next several years, she isn't going evil anytime soon and Frost is too cool a visual to not use early in a Firestorm show.
Sidenote: I've just started Arrow. I'm on ep 17 of S1 so I've not yet seen any of the episodes with Routh as Ray Palmer. However, there was that article with the shows producers who mention, and I paraphrase, "fans are going to be disappointed if the expect A.T.O.M. to be shrinking down". Really, then why be ATOM, a in name only concept does not excite me at all.