I know most people won't necessarily agree with that sentiment, but I cannot imagine Oda designing some members of the crew to have goals more important than other members. He spent years prior to the start of publication developing these characters and the longer the series goes, the more intertwined all of their dreams seem to be. I think that none of them can accomplish their goals without Luffy. Even Zoro.
Fitting symbolism associated with the cosmogonic cycle, outlined by Joseph Campbell, day and night or sun and moon motifs are used to describe the state of the One Piece world. Joyboy and his allies seemingly failed in their mission 800 years ago, but set a plan in motion to preserve the truth of the world for a future prophesied hero to someday incite a global restoration.
The sun set on Joyboy's era, but a New Dawn will someday rise. Pedro chose the name Nox for his crew because it is synonymous with night. For 800 years, the One Piece world has been in a darkened state, but the New Dawn represents a global restoration. In this long dark night, the moon is the symbol of hope - a reflection of the sun's light to prove that the light isn't truly gone from the world. The Straw Hats represent the coming New Dawn.
So if the world is asleep, dreams serve not only as a motivating desire for these characters. The pursuit of impossible dreams is one of the central themes of the series, but also perfectly fits with the sun / moon, day / night motifs Oda has chosen for the central narrative arc of the entire series. The quote from Roger which accompanies Dragon's introduction indicates the importance of dreams as a means by which adventurers are conducted across the sea in pursuit of freedom. "Inherited will. Dreams. The ebb and flow of the ages. As long as people hunger for freedom, these things shall exist."
The world of One Piece has not been free for a long time, but Joyboy left behind One Piece and his allies protected the truth believing that someday a group of heroes will arise to 'challenge the world' and unseat the dark forces which rose to take over the world in their era. Roger arrived too early, so he passed the message along.
I think all of the Straw Hat's dreams represent different sides of the global restoration which will follow the discovery of One Piece, the defeat of the World Government and Imu (the central antagonists of the entire series;not a pathetic, sniveling coward like Blackbeard), and can only be accomplished through being part of the crew of the Pirate King. I just think the connections are more obvious for some characters like Robin, Franky, Brook, and Jimbei.
Nami's dream is to map the entire world, which at first seems disconnected, until we ask the question why none exists. What if the world is not in the state it is meant to be? What if there is no map because it can only be drawn once the Red Line is broken apart and the world is set right. What if the same can be said of All Blue? All of the Island Whales have the same scars as Laboon, indicating that what we perceived to be unique behavior to Laboon is in fact innate to his entire species. That Island Whales repeatedly ram against the Red Line indicates its presence is an aberration. Brooks' dream of reuniting with Laboon requires circumnavigating the globe because the Red Line exists. Laboon was stuck at the Twin Capes, apart from his family, because of the Red Line. Brook's dream may thus represent not a single reunion, but setting the world right such that the conditions which necessitated an arduous journey to make the reunion possible are corrected.
There are Straw Hats with less tangible goals like Chopper and Usopp. Yet, I think we can already see how their dreams will manifest in ways tied to Luffy's. Chopper inherited Hiruluk's belief that no disease in the One Piece world is incurable and the minute we learned Roger was diagnosed with a terminal illness, lots of flags should go up that Luffy's life may need saving. Crocus could only help mitigate the effects of Roger's illness, but he was seemingly beyond cure. Yet, in the final months of his life, he met Whitebeard in a land covered in cherry blossoms, suspiciously similar to the mythic place where Hiruluk claims he was miraculously healed. I can only imagine Chopper's quest will culminate in curing the fatal illness which cut short the life of the Pirate King. Roger realized he was twenty years too early when he arrived on Laugh Tale and because of his illness, only had a year to wait. He realized the hero task of saving the world would fall to his successor on this basis. Roger's illness changed the course of world events. The former Pirate King could not sit idle and wait for the promised day because he simply didn't have the time and so he instead created the Great Pirate era with his final words, ultimately inspiring Luffy to take up his mantle.
Usopp's goal, to be a brave warrior, is admittedly vague at this point, but I think the role he's meant to play in all this will become much more clear through the Elbaf story arc.
And lastly, we come to Zoro. I don't think Mihawk will be his final opponent. And I absolutely don't think it will be someone like Shiryu. Rather, I think it will be the member of the Five Elders carrying Shodai Kitetsu. I think Mihawk's relationship to Zoro is very similar to Rayleigh (and Shank's) relationship to Luffy. Mihawk is pushing Zoro to surpass him and I believe that is for a reason. It's not good enough for Zoro to be as good as Mihawk. He needs to be better than Mihawk. Because perhaps there is an opponent Mihawk was unable to defeat.
Oda heavily hinted in volume 96 that Zoro is directly related to the Shimotsuki Clan of Wano. Zoro now carries Oden's sword Enma. Depending on how things develop, Zoro's quest to become the world's greatest swordsman could be very much tied to Wano's legacy dating back to the Void Century. I actually feel very confident in this idea as there seems to be quite a lot pointing toward Zoro being Wano's representative aboard the Thousand Sunny.
As for Carrot, I think the Straw Hats will ultimately have someone aboard the ship to represent each of the four groups charged with protecting the Road Poneglyphs. We already have Jimbei for Fishman Island and potentially Zoro for Wano. The Minks are waiting for the promised day just like the Fishmen. We already know what the New Dawn means for the Fishmen. We know why this global restoration is important for them. It means living under the true sun as they have desired for centuries. But why is it important for the Minks? In tangible terms, what does the New Dawn represent for them? The term implies they will be restored to their rightful place in the world. We know they have not always lived on Zou and that someday Zunesha's journey will be complete. But importantly, I think there is a hint in the fact Sulong is called their 'true form.' We saw in Clover's lab a globe with multiple celelstial objects. Right now Minks can only access Sulong on the night of the full moon, but what if this limitation didn't always exist? This is purely speculative, but the fact that the Minks have clung to this prophecy just as tightly as the Fishmen and Kozuki for centuries indicates that they are equally destined to play a pivotal role and that their people will be similarly restored through the New Dawn.
I made a couple videos where I talked more about this idea actually.
I think their dreams are still individual and will be fulfilled independently - but they're also one. They're all facets of the same quest and can only be accomplished together. I may be wrong about this, but I think evidence has continued to mount in favor of this reading the longer the series goes.
The fact is, Luffy is a chosen hero. At this point, there's virtually no escaping the fact nearly everything about not only Luffy, but his entire crew is special or unique in some way.