The Binding of Isaac: Afterbirth+ has been confirmed to be a new piece of paid DLC coming later this year. It adds in new items, new enemies, new bosses, redesigned content, and "a few surprises."

In addition to all of that content Afterbirth+ includes mod support. Users will be able to create custom runs, item placement, sound packs, scripts, etc with this DLC. The new content, save for the mod tools will be on PC, OSX, Linux, XB1, and PS4. The mod tools will only be available on the PC.

The blog also notes that those who want Afterbirth+ will need to own both Rebirth and Afterbirth. The blog entry includes a few additional details.