While Maxwell Lord, played by Peter Facinelli, may or may not be a villain, two supervillains will definitely be present: Livewire and Reactron.
The first is probably more familiar than the second to more casual fans of DC Comics. Livewire, as Leslie Willis, appeared first on Superman: The Animated Series, created by Bruce Timm. The character, after an accident with a storm, gains the ability to manipulate electricity - even become it, herself. She's just recently come back to the DC Universe, courtesy of a redesign by Babs Tarr for Batgirl.
Reactron, meanwhile, is a straight-up Supergirl villain, first appearing in her own book in 1983. A former soldier in the US Army, an overload of energy from another metahuman created his own powers, the ability to generate radiation. In one version, the character isn't just a figurative nuclear reactor, but a real one - with a Kryptonite heart, no less.
So far, this means there are pre-Crisis characters, race-swapped characters, characters from animation, characters from recent years; it's safe to say that when Greg Berlanti says, "We'll do it just like the comic books," he's very serious.