It appears like the original Binding of Isaac has not been forgotten as the game is getting a new DLC in near future.
The new DLC is being worked on by the co-creator and programmer Florian Himsl. He has announced that he is currently working on a new DLC for The Binding of Isaac: Wrath of the Lamb.
The said DLC will bring a Hard Mode, elite versions of several monsters, and in essence is similar to Rebirth with a ton of bug fixes.
Responding to a fan in YouTube comments section regarding the possible arrival date and the amount of stuff that is being added into the DLC, Himsl stated:
I am trying to add a mostly more enemy behaviors mostly. Will see what else I get the time to add. I always wanted to go back and fix these things. But my computer was laggy in flash and I kept making more mistakes then I was fixing bugs, but now I feel I have become a better programmer and also I have a new computer.
Sharing more details regarding the project, Himsl stated that currently he is the only one working on it, but he is hopeful that other creators will add to his ideas:
So far it’s only me! But if I do good they might add my ideas to Rebirth.
There’s no word how long it will take for DLC to release or how much will it cost, but we are expect to hear more in near future.