Pre-orders for the next set for 51st State are being taken now. It's called Allies and if you're a fan of the game, you'll want to get in now. There's two factions that are only going to be released as part of the expansion's pre-order.
From the website:
Even after the apocalypse, life forms have managed to endure within the remnants of the United States. Groups began to form, uniquely adapted to the new, harsh environment. Each one has its strengths, and you know that it would be in your best interest to be on good terms with all of them.
As of late you’ve noticed a few of these bands in your region. In the abandoned tunnels and underground structures, intelligent, stealthy ratfolk – the Sharrash – have been spotted. On the nearby stretches of road the large machines of the resilient Uranopolis have begun to roam, and further scouting revealed them to be in conflict with the Iron Gang who can occasionally be spotted darting among the machines, destroying and scavenging anything they can.
Dive headfirst into the never ending conflicts among the groups and try to deftly pull the strings from above. Increase your influence with the assistance of Sharrash, gain new capabilities with the aid of Uranopolis, or dispatch the Iron Gang to raze your enemies!