As of now, I believe that the Straw Hat Crew is the strongest non-Yonko pirate crew, with very reliable members who excel in battle, navigation, stealth, medical knowledge, archeology, and engineering…
They already proved themselves to be a real threat to the World Government and pirates as well, but as foes get stronger with each arc, Straw Hat members are ought to improve as well in what they do, especially in battle and fights.
As the way I see it, I think it will goes like this:
Luffy
Unlocks Gomu Gomu no Mi Awakening
Masters Advanced Conqueror’s and Armament Haki
Proficient in Future Sight (Advanced Observation Haki)
Unlocks Gear 5 which would need him to reach Advanced Conqueror’s and Armament Haki as well as Awakening his devil fruit, thus turning everything he touches into rubber.
Zoro
Masters Advanced Armament
Creates a Black Blade (like Ryuma and Mihawk)
Improves Observation and Conqueror’s Haki
Sanji
Unlocks Future Sight (Advanced Observation Haki)
Unlocks Conqueror’s Haki
Improves Armament Haki
Unlocks “Blue flames version of Diable Jambe” (Increased Heat)
Usopp
Kabuto has seastone coated on it (Increase range and power of shots)
Usopp will become proficient in Observation Haki
New variations in ammo