so this is my first time watching supernatural and im just on season 13, but i do know that Dean dies, Sam as a wife and kids, Sam dies eventually and they meet in heaven.

Now, Sam as a kid named Dean from what i saw and no doubt that Sam kept the Winchester last name.

So in my opinion, the spin off would be sorta like...the angels and demons finding out that there's another Dean Winchester who carry's the Winchester legacy. All the creatures that Sam and Dean defeated goes after Dean 2 bc they know that Winchesters are ruthless and now Dean2 as to figure out his legacy all while fighting these creatures by himself.

like i said, it's my first time watching supernatural and i haven't finished it yet so maybe this can't happen. But i would prefer this spin off lol