McMillen wants to keep most of the game a secret until "it's totally finished and I know what I want to do with it." However, he does note that "it's a play on my favorite game of all time" and that it's a roguelike game. Based on the teaser image, we'd fathom it's a dungeon crawling RPG where you battle your parents. "It should be done in about a month," McMillen adds.
The Binding of Isaac is being developed in collaboration with programmer Florian Himsl, and composer Danny Baranowsky (who also worked on the Super Meat Boy score). Although it promises to be "much smaller" than Super Meat Boy, it's definitely something worth keeping an eye on.