While I would watch endless hours of Winchester shenanigans in heaven, I also think they paved the path for a reboot.
There’s a Dean Winchester junior walking the earth. A Dean Jr. with the knowledge of the supernatural and a key to the bunker. Shouldn’t be too hard for him to bring Sam and Dean back, because something happens and he really needs their help.
Or, even easier scenario: something that only the Winchesters can solve happens, and Jack simply brings them back, asks them for help. Because, you know, while he isn’t hands on, he wouldn’t just sit back and watch the world get destroyed by whatever villain they cook up for a reboot. So he asks his dad’s.
Then there’s also Rowen, Queen of hell, and certainly powerful enough to revive them to help her out of a tight situation.
Plenty of possibilities. More than enough room to reboot.