The Binding of Isaac: Rebirth and Afterbirth do not officially support modding, but as with any even remotely popular game people have found ways to add their own little twist to the gameplay. If you browse around you'll find numerous mods that add custom bosses, balance changes, items, and other such things, but almost every single one of those mods is just that - a single mod.
The upcoming The Binding of Isaac: Antibirth, however, is less of a mod and more of a fully fledged, fan made expansion! There's a bunch of new characters to toy around with, ~90 new items to collect, ~16 new bosses to die against, ~4000 new rooms to explore, a new secret final chapter, and most importantly, an improved co-op mode. If you're wondering what all of this might look like in gameplay terms, wonder no more as the developers have also released a brief trailer. Have a look:
As for all of the new features coming in with Antibirth, here's what you can expect to see:
• new characters with unique playstyles
• 90+ new collectible items to help Isaac fight
• 18+ new trinkets to swap and strategize with
• 80+ new enemies to face one after another
• 16+ new bosses to kill or be killed by
• 4000+ new rooms to explore and survive
• A new secret final chapter for Isaac to endure
• Bigger and greater secrets than ever before
• Several new or enhanced features for local cooperative play
• A nearly full and original new soundtrack, evocative of classic Isaac
The Binding of Isaac: Antibirth is set to arrive on December 20th, and best of all, it will be completely free! So if you're interested in grabbing it as soon as possible, or just want to learn a little bit more about it, head on over to the official website.
And finally, it is important to mention that this is not an Afterbirth mod, but rather a Rebirth one. It can co-exist with Afterbirth (and the upcoming Afterbirth+), but your progress in the mod and Rebirth itself will have absolutely no impact on your main Afterbirth save. But if you enjoy all of the Afterbirth additions far too much to simply let them go, don't worry as the developers are planning to add support for all of the Isaac DLCs in the next couple of months. There is no exact date just yet, for obvious reasons, but its good to know that such a feature is indeed coming.