Although State of Decay is the fastest-selling original game in Xbox Live Arcade history, developer Undead Labs is still waiting for its sequel to be given the go-ahead by IP holder Microsoft.

An open world zombie sandbox title, State of Decay has sold over half a million copies since its launch on June 5, with half those sales occurring within its first two days on the market. That makes it the fastest-selling XBLA original game in the platform's history.

The proposed follow-up, codenamed Class4, would be an MMO according to Undead Labs.

"We cannot begin working on Class4 without a greenlight from Microsoft and a mutually agreeable contract," the game’s director of community Sanya Weathers told Eurogamer.

"We have to agree on where we think the technology is going to be in a few years (since the game will take a few years to develop); we have to agree that we have the same goals; and we have to agree on how we'll collectively run and support the final game.

"Those discussions are still in progress."

Despite massive sales, State of Decay’s launch hasn’t been smooth. Last week, the game’s first update failed to fix some of the bugs it was designed for, and a new patch is not yet available.

The developer also has its hands full with a PC port, which Undead Labs founder and executive producer Jeff Strain told Xbox would be available on Steam once mouse and keyboard support had been added.

“Beyond that, we're laying plans for the future of State of Decay,” he added.

“There's an incredibly sophisticated simulation engine driving the world of State of Decay – we call it FateEngine – and it gives us a platform to do much more with the game.

“For a start, I can tell you we're working on a sandbox mode designed to be a pure survival simulation. Beyond that, all I can say is we pay very close attention to what fans of the game are asking for, and their needs will largely guide our development priorities, so stay tuned.”