Gear 4 gives Luffy an insane power boost, but like Gear 2 and Gear 3 before it, it comes at a heavy price for use.
It uses up a lot of energy, making Luffy incredibly feeble as soon as he stops using it, regardless for however long he’s used it. So even after using a single attack, like he did against Big Mom (Kong Gun) and Cracker (Cannonball), simply deactivating the form causes him to immediately start eating through energy at such a rate that he quickly became hungry after having eaten as much biscuits as he could fit in his stomach, which we all know of Luffy, to be a hell of a lot. Now he does still have enough energy left to run and use his Gear 2 and Gear 3 attacks, but in a weakened state, almost like he’s exposed to Sea Stone.
It also uses up a lot of Haki. To maintain the form, Luffy has to use Haki almost all over his body to shape and control his elasticity, in so doing resulting in him having only a limited window of time to fight within before his Haki runs out – 2 hours at most so far – and forcing him to run for 10 minutes, until it replenishes.
These limitations will need to be overcome for Luffy to really get stronger, meaning we need to see Luffy develop a way to fight that allows him to remove these vulnerabilities.
Katakuri showed us that he had the ability to increase and decrease the density of his Mochi, via Block Mochi and Flowing Mochi, to make his arms harder and strengthen his attacks, as well as to make his body more fluid and malleable to dodge and maneuver freely.
This kind of ability would allow Luffy to substitute his use of Haki all over and instead change the density of his body parts to achieve the same thing, thus significantly increasing the length of time he can fight with Gear 4. But it would also increase the control he has over his body, as well as the strength of his attacks. He’d be able to transition from being almost as viscus as latex to as tough as vulcanised rubber, leading to a boost in his attack power just as Katakuri gained with his Block Mochi attacks. This is something that Luffy could definitely gain through Awakening.
Awakening could potentially also reduce Luffy’s energy use in Gear 4. By giving him more control over his body functions, his use of Gear 4 would be more efficient, although I could see Gear 4 being considered too powerful to allow it to remove all limitations, ergo I can see Oda deciding to keep the cost of using it high to some degree.
Another thing Luffy now needs to enhance is his Armament Haki. We’ve seen him honing his Observation Haki against Katakuri to reach the advanced stage and awaken Future Sight.
As we have seen in Wano, Luffy can now see the future as fluidly as Katakuri, as shown when he helped Hyogoro dodge multiple attacks from Alpacaman and Madilloman before they happened.
But now Luffy needs to attain advanced Armament Haki, which, if Observation Haki was any indication, should make basic Armament Haki barely effective against it. Recently, it is shown how Luffy is mastering Armament Haki in Wano.
Luffy has advanced his Armament Haki, allowing him to project it from his palm as well as progress even further by destroying objects he touches from the inside. While training to achieve the former, he unwittingly managed to progress all the way to the latter when he ripped death collars off of himself and Hyogoro. After training for a week under Hyogoro, Luffy now appears to be able to project his Armament Haki at will, as seen when he was able to blast a hole through a tree by projecting from his fist.
Luffy was able to use an even higher grade of this technique which causes the user’s Haki to reach inside the target’s body and destroy them from within.
It is unknown if Luffy is capable of destroying objects from the inside at will.
*Theory by misakghazaryan