The Binding of Isaac developer Edmund McMillen has posted a news (tease) of the upcoming game that he and other developers have been secretly working on for some time. Here are some quotes:
"James Id and I have been working on this secret project for a few months now and we can't hold our tongues any longer! The Facts: The Legend of Bum-bo is a turn-based puzzle RPG type thingy that's randomly generated."
While the developer presents the game as a new game, it is not clear whether the game is standalone or has some connection to The Binding of Isaac.
If you like to know more about the upcoming game read this blog link.