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
Gains Advanced Armament Haki (Rayleigh’s Technique revealed in 937)
Proficient in Future Sight (Advanced Observation Haki)
Advanced Conqueror’s Haki (like Shanks)
Unlocks Gear 5 which would need him to reach Advanced Armament Haki as well as Awakening his devil fruit, thus turning everything he touches into rubber.
Zoro
Unlocks Advanced Armament
Creates a Black Blade (like Ryuma and Mihawk)
Improves Observation Haki
Conqueror’s Haki (like Silvers Rayleigh)
Learns to cut fire (Fighting King)
Sanji
Unlocks Future Sight (Advanced Observation Haki)
Improves Armament Haki
Unlocks “Blue flames version of Diable Jambe” (Increased Heat)
Raid Suit:
-Invisibility
– Accelerators increase his speed and kicking force
– Armoured Legs (Like Steel)
– Floatation devices (Allows for flight)
– Cape (Sanji uses this as a shield)
Usopp
Kabuto has seastone coated on it (Increase range and power of shots)
Usopp will become proficient in Observation Haki
New variations in ammo
Franky
Shogun Franky becomes more streamlined, think Isaac from World Seeker
Franky upgrades his suit to incorporate Seastone into his fists
Unlocks Armament Haki
Nami
Zeus
Climatact infused with Seastone
Flight (By riding on Zeus)
Unlocks Observation Haki
Is able to create a thunderstorm over a country (Ultimate Move)
Nico Robin
Flight through Wings
Creates a giant Clone of herself (Full Body Clone)
Unlocks Armament Haki
Unlocks Awakening of her devil fruit thus making her able to create physical copies of everything she touches.
Chopper
Accessing the Monster Point without a rumble ball
Improves endurance
Unlocks Armament Haki
Brook
New Sword (A Meito- Personally I think he should get one of the 21 O Wazamono Swords)
Greater Ice Generation
Full Body Armament Haki (like Pica and Vergo)
Unlocks Awakening of his devil fruit, allowing him to manipulate souls of everything he touches.
*Theory by stealth_hawk