Well, chances are we will learn more about how Doflamingo knew about Imu, who was likely the "Treasure of Mariejois" which he talked to Law about. And Dressrosa was one of the countries that was built up above the sea level, in order to avoid the flooding, so it was foreshadowing that a bit. We did get "God Usopp" out of that arc, and that will probably play out in some fashion down the road. The Tontatta and their history of being enslaved may come up again. It's also where we first found out Sabo was still alive, and since it's the place he got Ace's Flame Fruit, it's kind of the functional start of Sabo's rise to fame in the world. It's also where we first learned about Law's backstory, as well as most of the lore about the Op-Op Fruit, which may play a bigger role later.
But, I have a feeling that most of Dressrosa's importance is going to be more "metaphorical" rather than "historical".
The way Doflamingo was ruling over the land as a "false hero"? That's a metaphor for the World Government and Imu. Doflamingo's power to literally "puppet" people with his strings? Turns out that's VERY similar to Imu's Domi Reversi power. Luffy disguising himself to hide his identity? Possibly a metaphor for the Nika Fruit disguising itself as the Gum-Gum Fruit. The Tontatta accepting offerings and trying to help out the people? That's likely a metaphor for the Devil Fruit Spirits working in the background to rebel against fate. The SMILE Factory? That's likely a metaphor for how the Ancient Kingdom produced actual Devil Fruits (or someone before even the Ancient Kingdom). Everyone forgetting about the gladiator Kyros? That's a metaphor for everyone forgetting about Joyboy and Nika, and thinking they're only a old fairy tale, too.
Dressrosa as a whole might be one big metahpor for the excesses of the Ancient Kingdom during the Void Century, although I think most of the islands that we've visited after the timeskip are the same way, just at different time periods. We might be going from island to island seeing what the Ancient Kingdom was like, during different time periods, metaphorically. We might just be going in "reverse" with Punk Hazard or Fishman Island being the most "recent" in the Ancient Kingdom Timeline.
Dressrosa is also another island who might be hinting at this "Underground Disease" that might be a long running plot point in the manga. Senior Pink's child dies of a fever. And, after getting caught in a landslide, his wife Russian went into a vegetative state. Mind, this is all AFTER they moved into a heavily forested area. So, a lot like with Amber Lead Syndrome in Law's hometown, Yorki's jungle fever, and Tree Fever in Shandia, I have to wonder if this was all caused by something underground leaking to the surface. I could see both Gimlet and Russian's health problems being revealed to be a part of this overarching "Underground Disease" Oda seems to be building up toward. Personally, I have to wonder if it's less about the trees and more about the trees being essentially planted on top of a mass grave. It's something to keep in mind.
Dressrosa is also very circular. Just like Long Ring Long Land. I could see Oda eventually revealing something about these "circular islands". Maybe they're a key part of some sort of worldwide machine or something? Again, it's another thing to keep in mind.
There existing a underground slave factory likely also tells us what things were like in the First World's Underground Mining Operation, hinted at in the Chapter 1138 Mural. I imagine that the Underworld where the First World mined the Forbidden Sun was a lot like Dressrosa's Scrap Heap, where the Toys toiled in secret.
The Toys are the biggest part of this, to me. I think it's likely there's another race that's been transformed against their will, much like the Toys in Dressrosa. And I'd say the Transponder Snails/Den Den Mushi are the most likely candidates for something like that. The Snails are really one of the weirder parts of this world that haven't really been fully explained. I wouldn't be surprised if the Snails are secretly another race that's been transformed against their will, like the Toys were. A slave race that EVERYONE in the world is guilty of using. Hiding right in plain sight. Keep a eye on the Snails.
So, there might not be a lot to concretely link Dressrosa to the bigger story. But, I think Dressrosa is actually pretty rich in allusions and symbolism to the larger themes in the story. Dressrosa likely gave us a lot of hints about stuff we'll learn later on. It might not be "The hometown of Joyboy" or anything. But, the history of Dressrosa will probably parallel the Ancient Kingdom pretty closely. At least in the broad strokes. Dressrosa actually gave us a lot of little details that have yet to fully pay off.
