The following blurb comes from a Q&A over at developer Edmund McMillen's website...
Q: Is this really the end of Isaac? no more dlc?
A: i did say many times this was going to be the last isaac DLC.. and at the time i strongly believed it. but there seems to be something pretty cool on the horizon, its creeping up slowly.. its familiar but a bit different and its being held up by a group of people… when i squint i think i can make out a… oh sorry i gotta go, there is someone watching me in my space ship.
Certainly seems like there's going to be something more in the pipeline for The Binding of Isaac. We'll keep tabs on the situation for any upcoming announcements.