Oda planned from the very beginning for each Straw Hat to have a specific part of the end of the journey devoted to them.
With Sanji, it seems like Oda long ago planned to have a Yonko who was obsessed with food, so that Sanji could have his defining moment as a chef in “beating her” with cooking.
Zoro is based off Oda’s original character Ryuma, the king of swordsmen. Coming to Wano will bring Zoro’s character full circle and I am certain this will be the arc where Zoro plays a major role in the land of swordsmen.
Robin will be the one to finally unravel the history of the world, and will be deciphering the final Poneglyphs.
The ultimate test for a navigator would be finding Raftel, the place no one has been able to get to. That would be Nami’s key role.
Franky will be making Pluton, and I am certain of it. Perhaps even converting the Sunny into Pluton. He comes from a long line of shipwrights entrusted with the blueprints to Pluton in case an evil enemy ever acquired the real thing. I am certain either the World Government or the Blackbeard Pirates will be discovering the Pluton in Alabasta, and Franky as the last successor of the Pluton blueprints will have to make a counter ship to combat it, as the line of shipwrights was always destined to do.
Roger died of an incurable illness. At this point, Luffy has been shaving away his lifespan repeatedly, and I am certain that there will come a point where Chopper will have to tackle his dream of “curing any disease” head on when he has to save Luffy’s life
Many believe Ussop’s role will be Elbaf, but I think there’s something more to it that would be a little complicated to go into….Brook meeting Laboon also ties up a loose end but that’s not the same as playing a key role in the final arcs of the story, and I have more ideas on how Brook will play a part as well that would be complicated to explain…
*Theory by pentothecap