What is happening in the final panels of 1043?
Luffy might just be smiling because he woke up and realized that he’s unlocked his awakening, giving him one more chance and one more move to fight Kaido. He thought earlier that Gear 4 was his last move and it failed, so this new development caused him to grin since he wasn’t expecting it.
I do think that the Gomu fruit did belong to Joy Boy before and that he was an awakened user. For rubber to melt, it needs to be heated a lot, so I’m guessing that the drum sound of Luffy’s heartbeat was because his heart beat became so fast that the rhythm changed in order to heat his body.
Zunesha must have heard this heartbeat many times in battle before (since he was Joy Boy’s nakama) whenever Joy Boy fought with his awakening, so that’s why he immediately recognized that sound and dubbed it “the drums of liberation” because Joy Boy fought and freed the slaves while in this form.
As for Luffy dying, I think that he did come very very close to death. Correct me if I’m wrong, but when a person comes near death, their heart rate speeds up abnormally since the body is going into a sort of CPR mode where the heart is fighting to get oxygen to the surrounding organs as fast as possible due to the organs starting to fail.
This is likely what happened to Luffy and the abnormal heart rate caused his body to heat so much and so fast that it unlocked awakening. This is why I think that the Gomu fruit awakening is so hard to achieve and why it has not awakened for centuries, because the user is not able to figure out how to heat their body to a temperature high enough unless they experience near death first. Once that happens, they’ll know how much they need to pump their heart to unlock it.
Joy Boy must’ve also experienced near death at some point to unlock his awakening since he must’ve been a repeatedly targeted figure by the 20 kingdoms that made up the current World Government.
What could be Luffy’s awakening?
I honestly think the idea of his awakening being simply making your surroundings into rubber is kinda weird since I can’t think of any moves you can do except making a giant bouncy castle. Maybe you could cause the enemy to loose their footing but that’s about it.
I do think his awakening could be melted rubber (like Katakuri’s was flowing mochi rather than just the rice called mochi) and he could multiply his attacks such as his gatling and pistol if he melts down objects nearby, re-forms it into a fist and applies hardening (with Armament Haki) to create another rubber fist.
This way he could make multiple fists out of his surroundings and pummel Kaido from every direction. Kaido’s future sight would be useless here since he may see it coming but there will be no room for him to evade such attacks nor will he be able to defend himself from every direction. Kinda similar to how Luffy pummeled Enel in Skypiea and his Mantra did not help him due to the overwhelming number of fists.
Theory by OneFriendship9222 (https://www.reddit.com/user/OneFriendship9222)
