First things first, I wanna see how a logia user can be defeated without having to face a natural enemy…
Anyway...
Luffy: I think all the moves no on will be the same, but upgrades through gear 2 and 3 or a combination of both.
Zoro: Man, this guy has demons to access. That means he spans anywhere from Japanese to Chinese, heck to even Indian mythology... my guess is, since he already created a One Sword Style element move, his next step is to do another one on Two sword style
Nami: Blizzard would be great. Also using creating real tornados and incorporating water into it creating cyclones. (If you ask me, cyclones should jack up DF users because its water...) It would be sick if she could create hail meteors! She wouldn't be strong physically but she'd be a force to reckon with on the elemental scale. Storm would have nothing on Nami!
Sanji: Since he can kick 12 times, I can see him coming up with electrical moves such as Static Kick Rub his feet at high speeds on the ground, and launch them at people paralyzing them. Same concept as Devil's leg, except he doesn't spin around, he stands there rubbing his leg. Couple this with Moon Walk he'll never touch the ground and lose the element!
Usopp/Sogeking: I await until he can absorb shocks on the impact/reject dials, and shoot the impacts damage at people. Since its invisible, no one will know what hit them.
Robin: In one of the movies, she displayed a gigantic arm and I hope she puts that into her arsenal. Also evolve her fruit to the point where she can create limbs even in the middle of the air. Couple that with gigantic arms, one won't know where the next strike will happen.
Chopper: I forsee the full control of monster point, and then later on evolving to a point where he creates a new stronger form.
Franky: This guy is overdo for some sort of beam. If he meets up with Vegapunk, I wouldn't be surprised if he became a transformer or the t-1000
Brooke: I want to see him use more instruments in his fights, like a cello, or a trombone. For comedic purposes, use the trombone to deck people with by using the slide, and use the bell to blow away enemies because of soundwaves. For his sword, I see him using on a grand scale the pistol shots that Ryumma used, except stronger.