Not redesigning the characters who are already build around their DFs..
Obvious. The man burns with a passionate heart of fire, now his body does too. At the beginning of Dressrosa, I was totally open to Sanji becoming the logia of the Strawhats and Ace's "will" being there right next to Luffy to help his journey.
- Franky : Gasha Gasha no Mi
Douglas Bullet assembles and reassembles all sorts of things like his submarine into a mech or the island into the golem. Pretty much perfect for Franky, who has more fun piloting toys rather than becoming those things himself. And maybe it's just me, but Bullet bears a strong resemblence to Franky in body and face already.
Big Mom is more or less a weather witch herself, her Jolly Roger surrounded by the elements she employs. Nami can fabricate all sorts of weather homies to manipulate the environment, attack her opponents, and store them in her staff for later. And plenty of slaves to serve Nami, she loves those.
Vander Decken had the ideal DF for a cowardly sniper. Usopp touches something one time, then can fall back and run away and continue sniping over and over until it's dead. Usopp can make use of his sniping talents to make sure his target is finally dead from a safe range.
- Jinbe : Hito Hito no Mi, Model: Daibutsu
Blue boy turns golden. Jinbe has the perfect belly to make a perfect Buddha. With his punches and thrusts he already sends out vibrations through water particles, this Mythical Zoan gives them the thrusting power to emanate large shock waves from his hands. I'm pretty sure if you cut off the topknot, Jinbe's hair turns into an afro anyway.
This one was tough. Zoro is a strictly physical beast who is unlikely to resort to gimmicks, but I don't think just any old Zoan would add flavor to his Santoryu fighting style. Since Zoro is a glutton for punishment, Urouge's unnamed DF turns him into a full on masochist. The more of a beating Zoro takes and the more blood he sheds, the more he gets rewarded. I'm not sure of Urouge just grows his size, basically a lot of suffering to just active San Juan Wolf's power, or changes his body and boosts his power in multiple ways. But Zoro using Ichi Gorilla and Ni Gorilla to boost the size of his biceps is the kind of modification I imagine naturally accommodates Urouge's ability. As Zoro can supposedly endure a lot of damage, the pay off in general here is tremendous.