The devs have already said that things like this wont be hapening bc its not canon becose even those with devil fruits can have the same abilities as the ones without them.
the only possible way to create an incentive for fruitless (or as you coll it haki fighting style) while keeping it canon is since you have more slots you'd be able to use more fighting style moves.
Maybe this will come true since there will be sub-styles and thats just meaning fruitless players can put more FS abilities in their hotbar but DF users wont be able to as most of them have already filled hotbars with FS abilities, race abilities and on top of that fruit abilities.
Even so fruitless is good right now, with kairo there are many advantages and im prety sure kairo will be getting another buff