The only rule is to not use the devil fruits of the other Straw Hat Pirates members. So in my opinion:
- Zoro: Neko Neko no Mi, Model Tiger/Panther. The only fruit I see for Zoro is a carnivore Zoan, to only enhance his physical strength without changing his fighting style. A feline would be perfect for him.
- Nami: Goro Goro no Mi or Soru Soru no Mi. Nami already uses a lot the lightnings and thunderbolts, it may be too overpowered for a member of the weak trio but I think that she would be able to bring out the best from the Goro Goro no Mi. The other option is Big Mom’s Soru Soru no Mi, Nami is already using Zeus but being the devil fruit’s user she would create her own Zeus/Prometheus and would be able to use them in a much more effective way.
- Usopp: Tori Tori no Mi, Model Hawk or Giro Giro no Mi. The hawks are famous for their sight, also being able to fly could give him a nice advantage position. With the other fruit (Giro Giro no Mi) Usopp would get an incredible sight, with an X-Ray vision. Also he would be able to read the mind, making his lies even more effective and exposing others’.
- Sanji: Netsu Netsu no Mi or Sui Sui no Mi. That was hard, after the obtaining of the Raid Suit and its invisibility, Suke Suke no Mi is no more an option. So I thought that Netsu Netsu no Mi could be a valid option, because it will make his kicks way more powerful and his Diable Jambe would reach extremely high temperatures. Also it would be useful during cook 😛 I also thought about Sui Sui no Mi. Sanji is a great swimmer and it would be a waste to completely lose this ability, with that he could still use his Blue Walk. Imagine Senor Pink but more powerful, faster and with the possibility of becoming invisible. It would completely confuse the enemy, not even letting him know what’s going on.
- Franky: Eustass Kid’s devil fruit. Franky is a cyborg, so it would be cool if he could be able to use a cheap version of General Franky (which can’t be used every time) by attracting metals. Also with his shipwright abilities I’m sure he would be able to find some creative uses of it. He could also have the Kira Kira no Mi.
I’m not giving one to Jinbe because as a Fish-Man he would probably become weaker and I’m trying to mantain the original combat styles.
I didn’t used some fruits like Supa Supa no Mi for Zoro, Mato Mato no Mi for Usopp and Buki Buki no Mi for Franky because even if those devil fruits could fit their fighting abilities, it will not be the same for their mentalities. I can’t see Zoro using a devil fruit to create blades and fight with them instead of his ones, Usopp using a cheat that would allow him to surely hit the enemy without even a bit of skill or Franky that would have basically the same issue as Zoro.