There have been some exciting revelations in the Theros world-building panel at PAX so far, and Jenna Helland has dropped one of the coolest: The identity of the other planeswalker besides Elspeth. He is, as it turns out, a native of Theros, and a Satyr: Xenagos, the Reveler, the hedonistic planeswalker specializing in riot magic.
According to Helland, the planeswalkers on Theros may be more powerful than the gods sometimes (they are, apparently, some of the most powerful beings on the plane). When Xenagos ascended, he felt insignificant compared to the vastness of the Multiverse. This, of course, didn't suit his arrogance, so he plots mass destruction and raging, multiverse-wide parties - bringing him in conflict with Elspeth, who is determined to preserve peace and not see any other planes die senselessly.
As for what colors he'll be, he's definitely red - Satyrs are typically green as well, but since there was just a red-green planeswalker in the last block, we're either looking at a monocolor Xenagos or else a black-red one.
Keep checking back here for further Theros updates!