Well, that was interesting. It kind of resolved the "Bad Place" storyline that should have been the Wayward Sisters spinoff. Unfortunately, the resolution is that all of the alternate Earths are being destroyed by Chuck-- he's essentially burning his rough drafts. Kaia turns out to have been alive all this time in the Bad Place and is saved by Sam and Dean when Jack opens a portal in defiance of Billie's orders. Another Reaper helps them keep the portal hidden from Chuck, but Billie does not take insubordination well.
And finally, Billie confirms the prior Death's assertion that Chuck can be reaped. By creating the universe, he made himself part of the universe, and so his book writes itself. Billie has a plan to end God, and the Winchesters will play the key role. Yep, this is the Third Testament and the end is nigh.
But then what? Somehow I don't entirely trust what Billie or the Big Empty Entity might have planned for the next phase. I suspect complications will ensue.