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: Hie Hie no Mi. This fruit allows the user to freely create and control ice and cold temperatures. This would be an invaluable skill for a chef. Many recipes require the ingredients to be chilled, and having that ability at his fingertips would be a huge asset to Sanji. It would also give him amazing versatility in combat, as he would be able to use ice as well as the fire from his Diable Jambe.
- Franky: Eustass Kid’s Jiki Jiki no Mi. 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.