I don't have any firm ideas, if only because it's only ever been mentioned in passing ONCE in chapter 643. But, let's explore some possibilities, just for fun.
The first thing that comes to MY mind is...I doubt that "Nakrowa" is actually the island's real name? I feel like this could be another Raftel/Laugh Tale situation. Chances are, for a island that's this old and lost to legend, the name has been "corrupted" and changed over the course of time. So, if it does come back into the story, chances are "Nakrowa" is actually supposed to be pronounced and spelled in a different way.
...I have NO IDEA what that could be. R's and L's can swap with each other in Japanese. And maybe you could change the "a" at the end to a "er" or "ar"? But, that doesn't give us anything particularly insightful.
Naklowa? Nak-rower?
Na-Crow-Wa? No-Crow-War? Maybe the island is where the Lunarians got eradicated? "Crows" being a possible derogatory term for black-winged Lunarians.
Necro-War? Maybe it's a island of zombies?
Nakulu-Wa? Nuclear-War? Maybe the island got destroyed by a nuke?
Knuckle-War? Maybe...some big fight happened there?
Na-Kuro-Wa? "Kuro" can mean "black" or "steel" in Japanese, so maybe the island has some connection to ancient metals?
I don't know, there's some possibilities here, but nothing particularly stands out as likely. Maybe someone else can come up with something.
The second thing that comes to my mind is that the "Island of Dreams" description makes me feel like it could possibly have some connection to the origins of Devil Fruits. Devil Fruits are the desires or dreams of the people who wished them into existence, after all.
Maybe Nakrowa is basically the Punk Hazard of the ancient world?
Post-timeskip, we saw that producing SMILE Fruits basically took cooperation and trade between multiple different islands. SAD chemicals were made in Punk Hazard. Then shipped to Dressrosa where they were sprayed on Trees in order to actually grow the Fruits. And then the SMILE Fruits were shipped over to Wano to actually get eaten. Chances are, this parallels what happened with Devil Fruits during the time of the Ancient Kingdom. There was likely some sort of trade route between different countries or towns or lands that allowed for Devil Fruits to be created. A entire "assembly line" for Devil Fruits.
So, I could see Nakrowa being where the Devil Fruit equivalent of the SAD chemicals or Green Blood was produced. A "Island of Dreams" where the Ancient people "ordered" their wishes to be turned into Devil Fruits in the first place. Maybe with some sort of giant "wishing machine" that starts the process of turning a desire into a chemical?
If Nakrowa does have some connection to Devil Fruit Trees, maybe Nakrowa could BE the Emerald City? "Emerald" for the trees. I don't know, it'd be nice to solve two long-standing mysteries at the same time. Even though, Oda DOES like to keep these ideas separate, usually. He really loves maximizing the number of locations and people, whenever possible.
Or, a alternate idea, maybe Nakrowa is more tied to the First World, rather than the Void Century or Second World?
Maybe Nakrowa is basically the "Olympus" for the Ancient Gods? The place where the original Nika, and Earth God, and Rain God, all hung out? A "Island of Dreams" because it's where mythical gods made their home?
Because, chances are, the ancient First World Gods probably also had ties to Dreams, in some way. Gods and Devil Fruits are probably related...somehow. Possibly both drawing powers from "Dream energy"?
Laugh Tale seems to have more ties to the Void Century than the First World. So, I could see Nakrowa ending up being more about the First World, farther back in history. And it'd give us a chance to get a backstory on the First World.
Maybe while the Strawhats land on Laugh Tale, we'll see Cricket find Nakrowa. And then we go back and forth as both of groups uncover more and more of the Ancient History? It'd be...a little unfortunate for Robin to not be on Nakrowa, if it is that important. But, Robin's more focused on the Void Century, anyway.
The fact that it's called a "island" is the only thing that makes me doubt this line of thought. Because if Nakrowa was ALWAYS a island...that probably means it was around AFTER the world started flooding heavily during the Void Century. Implying that it's more of a Second World thing. But, maybe Nakrowa WAS a island during the First World, and it's just been completely covered by the ocean by now due to all the flooding. Cricket is a diver, after all, so that would work, too. Hard to say.
So, I don't know. There are possibilities, but I'm drawing from VERY little. I do feel like we'll likely see Cricket find Nakrowa. But, I could see it as like...one page during a epilogue to the story or something. More of a story-seed to tease for a sequel manga than anything else. Nakrowa COULD be very important to the endgame of One Piece, I suppose. But, I'm not sure if I see the Strawhats ever going there, themselves. At best, I feel like there could be some interesting lore behind Nakrowa, even if it doesn't end up driving the plot itself. But, we'll see. I could be very wrong about that. Maybe I'm underestimating what Oda has planned for this island.
