So, taking into consideration all of the lore that Supernatural has put in place throughout the years. How a new death is chosen, how the supernatural hierarchy is laid out all of these things (including the season 5 Sam/ Lucifer and Dean/ Michael sword plot line) here is what I believe could be the ending for the show. THIS IS ALL JUST ME THEORIZING THINGS, I COULD BE COMPLETELY WRONG (most likely am)...
So as we know, The Empty is the only being who can kill death, however, Death is already dying due to the scythe cut that Dean did in 15x18. As we aren’t fully knowledgeable about what happens in the empty, either her wound will continue to fester, will kill her instantly (doubt), will pause the festering, or it will heal completely. Yet that still does leave The Empty (being) and Billie!Death to have a battle within the Empty (realm), but that does bring up the line “You made it loud” what The Empty (being) said to Jack once he re-materialized, and how every demon and angel since the beginning of time could possibly be awake now, trapped in their confusion of where they are, what happened and what is happening right now. Let’s say The Empty (being) kills Billie!Death, then that means by the lore Supernatural has put in place, a new Death will have to be put in place, this would be the most recent angel to have died...That would be Castiel.
In this area, I believe Castiel will become the next Death, then as Chuck!God has no control over him, Cas can then read God’s book and help the boys kill him because he would be able to know how the story/ show ends.
In regards to the God/ Amara situation; how he consumed her in 15x17, I think that power wise, Amara is stronger than God and so she will overpower him and free herself from him then aid in the boys taking on God and she could even possibly sacrifice herself because she isn’t as dark as she once was, that way getting rid of both seemingly light and dark from this world, creating a fresh “balance”.
In the Empty (realm), technically Archangels are still Angels, so in theory Lucifer, Gabriel and Raphael would be asleep in the Empty, but as they aren’t in the cast list for either of the last episodes I doubt that this theory is even close to possible. But anyway... We never actually had the fight between Dean/ Michael and Sam/ Lucifer (Michael was in ADAM in season 5, and Lucifer was in NICK in season 13) we never got the fight that Gabriel repeatedly stated was their “Destiny”, so they could possibly take God on, again I doubt it though.
This leads onto me thinking that the show could possibly thinking that Dean could become the ruler of Heaven (Michael could help him along the way, if Michael survives the finale). Sam could become the head of Hell (Rowena could help HIM along the way). not going to lie I don’t know about Jack...sorry. But obviously if Castiel does become the next Death, he can visit all of the realms including Heaven and have a relationship with Dean (fingers crossed). Maybe Jack could be the protector of Earth if everyone gets restored once God is destroyed.
AGAIN THESE ARE JUST ME SPURTING A BUNCH OF IDEAS THAT MAY OR MAY NOT BECOME TRUE IN THE SHOW. BUT I’M EXCITED TO SEE WHERE THE CREATORS TAKE THIS ENDING!!!