Poeden
I always saw the theories on "The Cars" and "Upside-Down" meaning Red Hood to be... major stretches.
"The Cars" did a song called "Magic", and Upside-Down could easily translate to "backwards" due to 'reverse'. Zatanna is a magician who speaks her spell incantations backwards.
And Red Hood would be another Batman character. I really doubt they'd go that route, though it's possible.
But the character roster currently has Scorpion on the Heroes side, which... isn't very logical. Like Lobo, it's likely he's only on the Heroes side because when it's uneven the Heroes -always- have more. He'll likely be switched to Villains, and Zatanna will add to Heroes.
Ed Boon explained the clues.
The Cars did do a song called Magic
Upside down are the lyrics for a Miller Band(?) song called Magic.
So forth and so on.
The next character is definitely Zatanna, it's been confirmed.
Still, don't be surprised to see Red Hood and Powergirl.
This hacker guy has been spot on! I'm willing to bet money that Red Hood and Powergirl round out the second set of characters. And I don't know if there will be a third. Though I hope there is.
Scorpion will probably get moved over when powergirl, and zatanna land on hero spots. And then be joined by red hood in the villians side.