Luffy needs one big thing out of a fruit, creativity, the ability to expand on its abilities in weird ways. To make his dreams come into reality, wether that's becoming a pinwheel or just stealing Soru. Therefore one fruit comes to mind, the Ope Ope No Mi, whilst Luffy wouldn't be able to use it for any sort of surgery it would grant him the creativity he needs. It'd let him create anything, and upon its awakening he could use R-Room to cover his arms creating something like a P-Room, Punch Room. Which would let him do things similar to gear 3, or if he places it over his whole body maybe he can try to do something more like G2 or G4.
Another alternative would be Nikyu Nikyu No Mi, having the ability to bounce around at high speeds would be invaluable, blasting out countless air bubbles as a move like gattling would be great. But more importantly, the ability to push away ideas is something Luffy would probably figure out by just not understanding it would be absurd.
Or we can go for the Mochi Mochi No Mi which just means he can do his old moves but a bit better but then not get G5 which means he'd probably just fail against Kaido. Unlike the other two where his fruit would be so different we wouldn't be able to properly guess if it was possible or not.
