Ever since the Straw Hat Pirates entered the New World, only half of them have shown off or received significant power-ups: Luffy has Gear Five, Zoro has Enma, Nami has Zeus, Sanji has his Raid Suit and genetic modification, and Robin has her Demonio Fleur. The rest of the crew hasn't developed any great new skills or abilities that they didn't already have after their two years of training. Now that One Piece is in its final saga, they'll soon be expected to hold their own in one-on-one fights with the top men working under Blackbeard, Shanks, and the World Government. If these Straw Hats want to have a chance of winning or even surviving, they're probably going to need to get some serious power-ups soon.
Before getting too deep into it, here are the ground rules for defining a power-up. It has to be more than a basic improvement over a character's preexisting skills; gaining something like Haki or greater strength should be supplementary to the real power-up. Preexisting skills refer to anything the Straw Hats had as of their return to the Sabaody Archipelago; they could have a hidden skill like Gear Four, but considering how long it's been since this arc, it's unlikely. Ideal power-ups include the reveal, discovery, invention, or creation of new techniques, attacks, weapons. equipment, or forms. It should also be something flashy that can be prominently featured in promotional material and merchandise; this isn't a narrative requirement, but it would go a long way in making it seem like a significant power-up. With this definition of a power-up in mind, here are the Straw Hats who need one.
Jimbei
Jimbei may be a master of Fish-Man Karate and Fish-Man Jujutsu. He may have had a long career and more life experiences than most of the Straw Hats. He might even be the third or fourth strongest crew member. However, he's also the newest aboard the ship and hasn't had any power-ups since joining. Looking at him like this, he's long overdue for one.
For Jimbei, his "improvement" could be the revelation that he's picked up more Fish-Man Karate and Jujutsu techniques. He could be shown to have all sorts of moves thought to be exclusive to other fish-men: Hack's Yonsenmaigawara Seiken, Aladine's Umidaiko, Hordy Jones' Shark Arrows, or even Shark Pack. There might also be some advanced or secret techniques that haven't been properly showcased in the series. Jimbei joining the main cast should mean a fuller exploration of fish-man martial arts. He might even develop new techniques entirely.
Chopper
Chopper has unfortunately languished during his time in the New World. Even with his Monster Point now under control for up to 30 minutes, he doesn't come off as the powerhouse he was when he first joined the crew. It's nice that he's developed new forms, but he hasn't made much use of any of them besides Kung Fu Point.
He might be able to improve by finding a way to mix his forms. For example, he could combine his Kung Fu Point with his Monster point in exchange for some of the latter's power. If he did something like this, he could go up against the mid-level members of a pirate crew instead of taking useless swings at the upper-ranking members. Better yet, maybe he can find a way to mix forms without sacrificing power, then he could fight the New World's stronger enemies with a chance of winning.
Usopp
Usopp has put in a lot of work in the new world. He helped get Seastone cuffs on Caesar Clown, knocked out Sugar, and used Tama's dango to convert several Beast Pirates, among other things. However, as far as straight fights go, he doesn't have many major victories.
The ideal way to power up Usopp would be to improve his ammunition. Currently, he relies on Pop Greens, which can be used for various purposes beyond simple attacks; they're similar to Hawkeye's trick arrows in this regard. They work well against weaker opponents, but if he wants to hold his own against upper-level pirate crew members like the Tobi Roppo, he'll need something that hits a little harder.
He also may need to improve his marksmanship. For comparison, the Blackbeard Pirates' sniper, Van Auger, can shoot targets from beyond the horizon line. It's currently difficult to imagine Usopp accomplishing similar feats.
Brook
Brook has shown some steady process with his techniques in the New World. He's consistently displayed new sword techniques strong enough to bring down whoever he's gone up against. His only big loss was against Big Mom, but many fans consider him the MVP of the Whole Cake Island Arc anyway. Regardless, he has room for improvement.
His sword skills are more or less top-notch, so perhaps he can improve his Devil Fruit powers. He's already found a way to instill the chill of the underworld into his attacks. Perhaps if he underwent a Devil Fruit Awakening, he could manifest even more of the underworld in the world of the living. He might even be able to conduct the souls of the dead like some sort of Soul King. Whatever it does, Brook's Awakening could make fans far more comfortable with him going up against the New World's stronger enemies.
Franky
Franky has been doing well in the New World thanks to the power of his giant robot, General Franky. Even without it, his own body houses a powerful array of weapons like his Coup de Vent and Radical Beam. With that said, it would only be fitting for him to seek upgrades to his tech.
Luckily for Franky, the Straw Hats' next destination is Egghead, an island said to be 500 years in the future. There, they'll have to contend with the Marines' top scientist, Dr. Vegapunk. Assuming they come out on top, this will be the best opportunity for Franky to add more advanced technology to himself, the General Franky, and even the Thousand Sunny.
Franky upgrading his tech wouldn't just increase its strength. It could also improve things like response time and energy efficiency. It could even entail new features. Or he could just get brighter Nipple Lights.