*Spoilers for Season 14*
The overall greater struggle in Supernatural of Sam and Dean has been to escape the cycle of the constant high stakes fights and dangers.
I think the show could end in a few ways, but I don't want to list everything out in my head.
I'll just focus on how I think it should end and how I want it to end.
How I want it to end:
Sam and Dean encounter other Sam and Deans from across the multiverse. Seeing the "poorer" or "less entertaining" versions of themselves in order to learn why Chuck is interested in them. What makes them his "guys".
* We see versions of Sam and Dean from 2014 version of history where Sam was possessed by Lucifer and wearing a white suit. Kills Dean when Dean sacrifice's his team to shoot him with the colt.
* We see a version where Dean stayed a Demon.
* We see a version where Sam stayed souless.
* And we see versions of Sam and Dean who "played their roles" and said yes to Michael and Lucifer. The angels are repressed and our Sam and Dean ask them why they said yes.
Through all the alternative version of Sam and Dean, we learn that God likes are Sam and Dean, because he stopped being able to tell what they would/could do. They kept finding ways out of the corners he pushed them into and just opted to keep throwing storylines at them and try to get them to follow his themes and arcs.
Eventually, Dean and Sam take God's place, so to speak. Either a one-time deal or permanently, they get powered up to God level. They use the power to cure any existing monsters back into humans (as applicable), the remaining monsters (born that way, or entities like demigods/pagan gods) get killed instantly (Thanos Snap style).
All demons are then pulled from Earth and trapped in hell. All ghosts are forcefully moved on and ghosts are no longer of staying on Earth passed their death or just never can interact with the world.
Effectively, this removes the need for hunters. No one has to be trapped in that kind of life.
Sam and Dean then run Heaven, keep the angels in line, keep Heaven powered up so the souls can stay.
They make Cass the new guardian of The Empty he can finally get some rest and Jack stays in Heaven and gets to live with his Mom with visits from Sam, Dean, and Cass.
How I think it should end?
Sam and Dean don't just die, their souls are eaten by Jack. As part of a plan with Cass's support, Jack kills God allow Billie to reap him.
Jack is the only one left on Earth and continues to hunt the way the Winchesters and Cass taught him. He has no more power (for awhile). And Cass goes to The Empty.
The major theme being that they pass things on to Jack as their successor. Legacy has been a recurring theme of Supernatural and making Jack pick up after them, I think, is critical to the message of family and paying it forward.