What do I do now?

[Dr Six, corgi battlemaster]

I beat both Satan and Isaac, I beat Satan again as Judas because I thought maybe that would get me an alternate ending of some sort. What am I supposed to do now? Thank you in advance.

[The Renderer]

Oh boy. You've barely scratched the surface.

Complete the post-it notes for all characters (beat everything with all characters, some of the things you need to beat you haven't even found yet)
Complete all challenges
Unlock new characters
Unlock all items, trinkets etc.
Find all items
Do the greed mode stuff

Generally try to get all achievements, if you have done that, you kind of "completed" the game.

[Nazemec]

General idea of the Isaac/Satan step: kill each one several times. At some point, you'll get an unlock to go further (well, two unlocks, one for each).

[HeraldOfOpera]

WARNING: SO MANY SPOILERS

Beat Isaac 5 times to unlock The Polaroid, then beat him again while holding it (it'll be guaranteed from Mom).

Beat Satan 5 times to unlock The Negative, then beat him again while holding it (it'll also be guaranteed from Mom; under normal conditions you can't pick up both but they'll both drop).

After doing at least one of the above, bomb the angel statues in Angel Rooms at least twice in the same run while doing it again (or the other one, if you want to save a bit of time).

Complete the first six floors in 20 minutes or less by the end of the Mom fight in Depths/Necropolis 2, or use Mama Mega on that sixth floor. Go through the door, take whatever item you think looks best, and hold onto your butt.

Beat all of the above bosses as every character you have, ideally on Hard but only one character really needs to do that and you probably haven't unlocked him yet. If you buy/bought DLC, beat all of the below bosses too.

If you decide to buy Afterbirth:

Greed Mode, duh. The fact that you didn't mention it is why I'm assuming you don't have the DLC yet. You'll want to eventually get 1000 coins into the special donation machine at the end of it, and you'll also want to complete it with Azazel.

Assuming you've beaten Mom's Heart 11 times already (getting 2 Satans and an Isaac without doing that is unlikely), complete the first 8 floors in 30 minutes or less by the end of the It Lives fight that has long since replaced that one, or use Mama Mega on its floor. Go through the new door, preferably with at least 7 keys, grab the hilarious amount of free stuff and the shop, then realize just a second too late what that sort of generosity usually implies in games.

If you decide to buy Afterbirth+:

Greedier Mode is a thing now, and unlike normal Hard Mode it actually has separate unlocks.

Assuming you eventually beat the nasty Afterbirth boss I sent you in ill-prepared for, go through the portal at the back instead of the regular continuation; it'll occasionally appear after any of the non-Greed bosses either of us have mentioned by now, but this is the only way to guarantee it. You can also leave and re-enter the boss room if you bought + before doing the initial kill.

Assuming you did the Satan thing way back in the second paragraph, beat the boss of the first floor in under 1 minute, then go back to the starting room and drop a bomb wherever. Grab the thing that drops, and never let go of it. Then do that a couple dozen more times until you get good enough to make it through the Mom fight while dealing with the consequences. After that, take your newly-restored shovel to the seemingly-empty dead end in the floor after Satan, and activate it in the weird-looking spot in the center. You're welcome.

[Calloway]

Nice, you finished ~10% of the game. Both the Satan Path and the Isaac Path goes even further. After Cathedral/Isaac comes The Chest with ??? as the boss, after Sheol/Satan comes The Dark Room with the Lamb as the boss.
Have you beaten the Boss Rush? Have you unlocked/beaten Hush? Delirium? Have you beaten Greed/Greedier Mode? Unlocked all of the 15 Character? If I remember correctly there are 35 challenges. There is a lot to do my friend.

You also unlock something new everytime you beat one of the followeing with one of the characters: Moms Heart/It Lives (unlock only on hard mode), Isaac, Satan, ???, The Lamb, Boss Rush, Hush, Delirium, Greed Mode, Greedier Mode, Delirium and one special unlock if you have beaten everything with one character on Hard mode (so just always play hard mode if you dont want to do some of them more than once). Thats 12 unlockables with each character. (Asuming you have both of the Afterbirth DLCs) (EDIT: I forgot about Mega Satan, thats one more unlockable with each character)

If you are unsure how to unlock any of the extra chapters, secret Characters, some of the challenges or anything really, take a look at the IsaacWiki page: https://bindingofisaacrebirth.gamepedia.com/Binding_of_Isaac:_Rebirth_Wiki


Happy hunting

[Dr Six, corgi battlemaster]

Thank you all very much, you've been very helpful!