The ending for Game of Thrones Season 7 episode 1 in showing Dragonstone being a massive castle is one in which I do not think it being a coincidence. Although we have seen glimpses previously with Stannis occupying the keep, Season 7 seems to increasing the importance in having various characters spend a significant amount of time there with Daenerys making it the core of her operations to take back the Seven Kingdoms. Yet besides the Dragonglass that Samwell suspects the island to hold a great deal of, what else will Dragonstone be significant for in the upcoming seasons?
I suppose that one idea that has been making me ponder lately is how Davos Seaworth had at one point saved Stannis in the past by bringing onions (and potatoes I believe) in smuggling them through during a siege. Much of Game of Thrones revolves around foreshadowing, which as we see quite often ends up becoming a situation where one scene will eventually mirror another. A speech in the past can easily act as a fortune telling for something that will happen in the future. And history repeats itself, but not by coincidence.
Because Dragonstone is composed of Dragonglass/obsidian, it becomes a formidable bastion against the White Walker army. There’s been talk where people have compared the introduction of Game of Thrones season 6 to season 7 on the east side of the wall, pointing out how the sea becomes frozen. In addition, the Hound foretold his own vision that leads us to believe that they will all end up around Eastwatch where Tormund and the other Wildlings are manning the abandoned castle to potentially hold off the army of the dead. If the Night King is gathering his forces there and the introductions are not coincidence in terms of their difference between season 6 and 7, then it’s possible to conceive the Night King might be freezing the sea in an attempt to go around the Wall itself.
If the Night King and his forces have this capability to convert sections of the sea and deep waters into ice, then perhaps if Westeros becomes overrun by this army, then they may have to flee to islands for temporary refuge. That makes a place like Dragonstone a critical area for a hold out. Perhaps, what we may see is large numbers of refugees eventually fleeing to Dragonstone to wait out the storm of the Night King. But supplies may dwindle leading someone like Davos to once again perform his duties as the Onion Knight.
In the meantime, perhaps the refugees will tear down the castle for armaments in last stand type of deal against the White Walkers. I do not know for certain whether any of this would happen but I do feel strongly that the composition of the castle and Davos Seaworth’s past experience of smuggling will once again make their rounds into the story in this type of manner.