So far, one of the most intriguing additions that Luffy has made to his powers is the usage of "Gears," which simply refers to different modes of power that he accesses by making use of his Devil Fruit. In One Piece's Enies Lobby, Luffy came up with Gear Second, which increases his power by pumping blood through his body faster, and Gear Third, which sees him inflate his bones to generate more power. After the timeskip, Luffy managed to create Gear Fourth, which made use of inflated muscles and Haki to generate power. Interestingly, it's been seven years since Luffy's last major Gear power-up and it's entirely possible for something new to come up soon in One Piece's Wano Country arc – Gear Fifth.
One of the biggest questions when it comes to speculating about Luffy's Gears is whether there is even going to be a Gear Fifth in the first place. After all, Luffy has just managed to improve his control over Gear Fourth and might not be ready for the next step just yet. While there is some truth to that thought, it's entirely possible for Gear Fifth to exist, depending on what fans think Gear Fifth will do. Luffy's gears work in unique ways and make full use of his Devil Fruit power every single time. With Gear Second, Luffy takes advantage of his rubber body to sustain an incredibly high blood pressure making blood flow much faster in his body, becoming stronger by the sudden doping-like effect. In Gear Third, Luffy simply inflates his rubber bones to generate as much power as he can, and Gear Fourth makes use of high-level compression thanks to Haki combined with the Gomu Gomu no Mi.
Evidently, there is room for Gear Fifth to exist for Luffy as he hasn't mastered every aspect of his Devil Fruit power just yet. For Luffy, one big power-up is still missing and that's Devil Fruit awakening. It goes without saying that once Luffy awakens the Gomu Gomu no Mi, his power will jump yet another tier higher, seeing what it has done for the likes of Eustass Kid and Trafalgar Law. Just like them, Luffy will likely be able to use his Devil Fruit in crucial moments and make use of powers that go beyond just a rubber body. As such, Luffy's Gear Fifth could be the form in which he solely focuses more on utilizing awakening.
If Luffy eventually awakens his Devil Fruit, the powers of Gear Fifth will likely have to do with it. For instance, Luffy made extensive use of Haki in Gear Fourth no matter which form he relied on. Similarly, with Gear Fifth, he's likely to make use of the new powers that he gained and that is awakening. To understand what Gear Fifth would do, fans need to understand what Luffy's awakening would be like. While the most basic ability of Luffy's fruit will likely be to turn everything around him into rubber, there's definitely more to it. By awakening his fruit, Luffy will also likely gain total control over the properties of rubber and also the ability to alter some of its properties. For instance, by toughening it up, Luffy could make use of vulcanized rubber and use it for extra toughness, which would help him both offensively and defensively.
At the same time, Luffy could also make rubber similar to resin and thus gain extra mobility. Furthermore, using the resin-like property of rubber, he could even make use of a logia-like power that would make him intangible, akin to Katakuri. After all, Whole Cake Island made sure that the fans understood how similar Luffy and Katakuri were in terms of power. By gaining these two abilities and spreading them to his environment, Luffy could essentially create Gear Fifth.
In theory, Gear Fifth is entirely possible and would see Luffy make use of vulcanized rubber and, at the same time, logia-like powers of intangibility when needed. All that, when combined effectively with Color of Arms Haki and the Color of Supreme King Haki would make Luffy nigh unstoppable. Since he's already strong enough to fight Kaido, the new Gear Fifth could give him some better techniques and make him much stronger than before, pushing him even beyond the level of Four Emperors and allowing him to be the strongest in the series by far. For now, Luffy hasn't used Gear Fifth but his fight against Kaido is yet to conclude and after Kaido, there are several other battles to awaken this power in.