Oda always gives small hints leading to the main events. He showed us that in order for Luffy to win, he has to improve his Observation Haki to Kakaturi’s level. That means he should predict the future and dodge attacks subconsciously.
Luffy managed to almost achieved that. We saw how Luffy dodged all Kakaturi fists. However, Luffy couldn’t attack him without taking a hit because Kakaturi was able to predict his position in the future and attack him easily.
What Luffy needs is a form that is very fast and makes it harder for his enemies to attack him even if they can see the future. Luffy’s body movements will be very hard to pinpoint in the future. And even if Kakaturi managed to attack him at the exact right moment, Luffy’s body will be flexible enough to dodge the attacks without Luffy even trying. Similar to Kakaturi shapeshifting his body.
We all saw how Luffy hit Kakaturi because he was fast. I think Gear 4th: Snake Man will give Luffy this ability. The issue with Gear 4th: Bounce Man was that Luffy’s body was exposed to Kakaturi’s attacks since he could predict the future.
*Theory by dare_dick