The Binding of Isaac's development blog updated recently with a generally unexpected and rather vague announcement of a new title, The Legend of Bum-bo. Very little has been revealed of the new project, seemingly by intent, however the active engagement of many The Binding of Isaac developers including the prolific Edmund McMillen has been confirmed.
Despite the development team's previous difficulties in getting some of their newer projects off the ground, The Legend of Bum-bo was confidently announced as a new "turn based puzzle RPG type thingy" that has apparently been in development for several months now. Information on the 'project' is sparse at-current, including its very nature as a product - the announcement did not in fact clarify whether The Legend of Bumbo is an entirely new title from McMillen and co. or merely an expansion project for The Binding of Isaac: Rebirth, similar to the original version's Wrath of the Lamb DLC. The situation becomes particularly muddled when considering that the announcement appeared on the official Binding of Isaac development blog rather than the main developers', however from this it can be generally assumed that The Legend of Bum-bo will have some relation to The Binding of Isaac, tenuous or otherwise.
Of note is that the accompanying artwork and logo contained in the announcement seem to feature an art-style heavily reminiscent of the original The Binding of Isaac rather than the game's 2014 remake, The Binding of Isaac: Rebirth. Although this is unlikely to be a sign of particular relevance to one title over the other however, as many of McMillen's earlier flash game titles feature similar styles. This may, however - in addition to the announcement that the game will feature 3D elements - suggest a significant distancing from either title despite the nature of the announcement.
For fans of the remake's soundtrack, composers Mathias Bossi and Jon Evans have been confirmed to return for audio design in The Legend of Bum-bo. It would appear however that the team's working relationship with the original's composer, Danny Baranowsky, is even less likely to return following this announcement.