The climax of the Wano Arc in One Piece saw the series' protagonist, Monkey D. Luffy, go up against and defeat Kaido, one of the strongest individuals in the world and one of the former Four Emperors of the Sea. Over the course of his time in Wano, Luffy fought with Kaido a grand total of three times, with each successive clash providing him a great deal of insight into how he could advance his fighting capabilities. Although Luffy was already quite powerful during his first fight against the captain of the Beasts Pirates — having beaten the likes of Doflamingo and Charlotte Katakuri — he was defeated in almost no time, which made him grasp the gap in power between them. This pushed him to train intensely, working on his advanced Armament Haki, known as Ryuo in Wano, which would allow him to damage targets from the inside rather than inflicting damage at the surface, in order to bypass Kaido's tough skin and scales in the dragon form granted to him by his Devil Fruit.
While this did aid him during their second clash, it was only the revelation of the ability to coat oneself in Conqueror's Haki, that saw Luffy finally begin to close the divide between him and his adversary, and proceed to fight him on equal footing. Lastly, the Awakening of his Devil Fruit, which was unveiled as a Mythical Zoan known as the Hito Hito no Mi, Model: Nika, gave him the final boost in power he needed to overcome Kaido's incredible might, as he fought with a degree of freedom that had not been seen before. At this current juncture in One Piece, Luffy's strength is shown to be virtually equal to that of an Emperor and perhaps even on par with the Admirals, but he still has a ways to go before reaching the heights of Shanks, who at present, is likely the most powerful character in the series till date. Hence, it is fairly certain that Luffy will still need one or two more boosts in strength before finally taking on the likes of Blackbeard, or the strongest fighters in the Marines and World Government.
Mastering Gear 5
Described by Luffy himself as his "peak," Gear 5 afforded him a massive boost in speed, power, and durability, with healing and regeneration on a level that is absolutely ridiculous. The malleability of Luffy's rubber body also reaches new heights, as he was able to withstand direct attacks from Kaido's Hassaikai, stretching to exaggerated extents in order to absorb the force exerted on him but subsequently returning to his natural state in almost no time. This regeneration is active at all times, even when Luffy is unconscious, indicating that it is his body's base state in Gear 5. Luffy can instantly switch between his Gear 2, 3, and 4 forms at will, and even turn himself into a giant. Beyond all this, his greatest power up comes from his ability to modify his surroundings into rubber, limited only by his own imagination. Luffy was able to stretch the very ground itself to block Kaido's Blast Breath and redirect it towards his opponent, showcasing the obscene and almost cartoonish nature of his Devil Fruit Awakening. He can also apply this to intangible substances such as lightning, which opens up many doors in terms of how he can use his newfound abilities.
In spite of all these benefits, the glaring issue with Gear 5 is the sheer amount of energy it consumes when activated. During the fight with Kaido, it is fair to assume that Luffy was in this state for a few minutes in the closing exchanges of the fight, hinting that he is yet to master it completely. Even after this short duration, he experiences intense fatigue and exhaustion on transforming back, and appears to visibly age in the process, even if he can trigger the state once again by altering his heartbeat to match the Drums of Liberation. In this scenario, the longer a battle drags on, the more of a toll it will take on Luffy's body, and the exhaustion will probably compound with each repeated use of Gear 5 in quick succession. Luffy's body still needs to get used to the strain of using the form — much like he did over the time skip with Gear 2 and Gear 3 — to be able to enter Gear 5 at will without wearing himself out entirely. The form might have been extremely effective in finishing off Kaido after a long and drawn out confrontation, but Luffy definitely has a lot of work to do in case he needs to use it against even more powerful adversaries like Shanks, Blackbeard, or the Admirals, in extended battles.
Leveling Up His Haki
Over the course of his last two battles against Kaido, Luffy came to understand the true potential of Conqueror's Haki, and learned to coat his own attacks with it, boosting the offensive power of his attacks to new heights. Kaido himself stated that Devil Fruits and all other power ups were futile in the New World, as Haki triumphed over all. This can also be seen in the case of someone like Shanks who relies primarily on his swordsmanship and Haki, much like the former Pirate King himself, who also never ate a Devil Fruit. Luffy's initial use of Conqueror's Haki coating was described as crude and amateurish by Kaido, even if it allowed the Straw Hat captain to fight on level terms with him. This changed by the time of their third fight, where Kaido finally acknowledged Luffy's strength after taking considerable damage from his Conqueror's Haki infused attacks, indicating that his use of the ability was now nearly on par with that of an Emperor of the Sea.
Nonetheless, Luffy still has not attained power that can rival someone like Shanks, Roger, or even his own grandfather Garp, who have all displayed a degree of Conqueror's Haki coating that can level entire cities or battlefields in a single strike. In this vein, aside from his already overpowered Devil Fruit ability, Luffy will have to undergo serious training to learn advanced Haki in the style of Shanks' level of Future Sight, Conqueror's Haki coating, and even his Observation Killing and ability to project his will over long distances. It is possible that Luffy might acquire these skills after training with or clashing against Shanks in the upcoming Elbaf Arc. With these upgrades to his arsenal, Luffy might finally be able to challenge the world's strongest in one-off battles, and charge towards his dream of laying claim to the One Piece.
The Voice Of All Things
Lastly, one of Luffy's abilities which is yet to be explored in its entirety is his capacity to hear the Voice of All Things. In the past, the PIrate King, Gol D. Roger was shown to possess a similar ability, being able to hear the voices of the Sea Kings, much like Luffy did at the time of the Fishman Island Arc. Additionally, Luffy was also able to hear the giant elephant Zunesha, Joy Boy's former companion, who carries the nation of Zou on its back. It is worth noting in this case that Luffy was unable to understand what the elephant was saying, unlike Kozuki Momonosuke, who was able to hear its voice and comprehend the information being relayed. Whether this ability will manifest as the power to command Sea Kings, rally armies and allies, or control large numbers of individuals and creatures is yet to be revealed, and the final saga of One Piece may yet hold answers to this and many other questions.