Luffy's strongest ability: making those around him into his allies. Everyone in his crew will be top 1 in his domain. Chopper will be the best doctor, Nami will be the best navigator, Sanji will be the best cook. What is Zoro's ambition? Only strength. First Nika was shown with a sword. And Zoro is Luffy's sword. Luffy will be the strongest because he will have the world's strongest weapon/sword, which is Zoro.
@nik87 @Salah WG
I don't think it's about the Sword, each SH's Purpose is to Create an Ultimate Version of something that would solve Humanity's Problems.
For example, Chopper isn't gonna Create a Cure for every single Disease, He will simply Create a Single Cure that works on them all, which will also make Doctors Obsolete,
Same goes for every other SH, Nami will Create a Map that is applicable anywhere, and Franky will Create a Ship that endures any Journey, and Jinbe will Create a Guide (Could be a Fish) that can move/direct the Ship on any Sea, and Robin will Create a Book, and Sanji will Create a Food, and Brook will Create a Song ... etc
One Creation to Replace all others, and all of this together, what does it do? It means you don't need a Doctor anymore, you don't need a Navigator anymore, you don't need a Helmsman anymore, or Cook or Historian ... etc what does this do? It means "Anyone" can be a Pirate, Luffy is basically forming the Crew & making the Journey that no one will need to go through anymore, the Journey/Crew to End it all, you won't have an Excuse anymore, you can enjoy any Adventure you want without lacking anything necessary for it,
Which brings us to Zoro, what will be his Creation? Since his Main Job is to Fight, then his Creation should make Fighters Obsolete (Basically creating a World where someone like Kuina can't exist anymore, a World where anyone can be Strong), and thus He will Create a Technique to Rule them all,
Zoro's entire Fighting Style is about coming up with Techniques for each Situation, Target, Opponent ... etc
He doesn't just randomly Attacks, He needs to carefully pick which Technique is suitable best (Similar to Doctors needing specific Cure for each Disease or Health Issue), but Zoro needs to do the Impossible Task of Creating a Single Technique that can Work "Anywhere",
So Zoro's Contribution to "Impossible Dreams" is to go beyond the restrictions of Strength & Stats & Weapons, it's like an Attack that is simply "Fated" to Win regardless of how you execute it & regardless of what you're up against, you don't need to worry about your Stats or Sword-Style anymore, a Feat that makes Zoro not only Surpass Mihawk (Who counters any Technique created in History so far), but also Surpass Death itself, and make his Name reach Heaven indeed