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.
Right now Luffy has finally awakened his devil fruit and has mastered all three types of Haki and its advanced applications. At this point it really looks like Luffy has reached Yonko level.
But what about the other 9 Straw Hat members? I think it will goes like this:
Masters Advanced Conqueror’s and Armament
Creates a Black Blade (like Ryuma and Mihawk)
Unlocks Future Sight (Advanced Observation Haki)
Unlocks Conqueror’s Haki
Improves Armament Haki
Kabuto has seastone coated on it (Increase range and power of shots)
Usopp will become proficient in Observation Haki
New variations in ammo