WARNING: The following contains spoilers for One Piece Chapter 1044, "Warrior of Liberation," by Eiichiro Oda, Stephen Paul and Vanessa Satone, available in English from Viz Media.
After 25 years of One Piece, the true identity of Monkey D. Luffy's Gum-Gum Fruit has been revealed -- Human-Human Fruit Mythical Zoan Model: Nika. For the longest time, the World Government hid the Fruit's real name. Now that Luffy has undergone an Awakening and given his new powers a test run, the truth can no longer be kept secret.
Apparently, Luffy's actual ability is to invoke cartoon physics. His rubber body is just an extension of this power. By activating Gear Five and assuming the form of the Sun God, his personality, his fighting style, and even his entire aesthetic are all greatly altered. If he really can bend the laws of physics like a cartoon character, then there's no telling how broad his skill set can become.
To clarify, applying cartoon physics (aka toon force) is a sort of meta-ability in fiction. Naturally, cartoon characters use it all the time, but when it's presented as a unique ability, only the character(s) granted the ability can apply it. This is a rare ability in anime, but some notable users include Panty and Stocking, Toon World cards (Yu-Gi-Oh!), Haruko Haruharu (FLCL), and Nui Harime (Kill la Kill). Now, it seems as if Luffy can count himself among this handful of anime characters who employ cartoon physics where others can't.
It's difficult to imagine a specific ability for invoking cartoon physics in a story like One Piece because the series is already so inherently cartoonish. Characters' eyes pop out of their heads, their jaws drop really low, their teeth grow back after being knocked out, and they can survive a lot of things normal humans can't. The Western influence in One Piece is obvious.
However, Luffy has done a lot of things throughout One Piece to come off as particularly cartoonish. Naturally, his rubber body allows him to stretch and inflate by blowing into his thumb. He also tends to eat tons of food, grow obscenely bloated, and burn it all off instantly. He's also somehow able to bounce his Gear Four attacks off the air; Kaidō even calls him out on this. Most of these unrealistic feats remain exclusive to Luffy even after over 1,000 chapters and episodes.
To further emphasize the cartoonish nature of Luffy's awakening, his entire aesthetic has been altered for Gear Five. Many of his features have been simplified and his lines have been smoothed out. The exaggeration of his movements is shown in the way different parts of his body inflate; a similar phenomenon can be observed when Popeye eats spinach. While plenty of characters in One Piece have eye-popping, jaw-dropping moments, Luffy's are taken up to eleven in his new state; the only character to rival him in this department is Buggy, and he needs his own Devil Fruit to do it. All of these things go together to make the Straw Hat captain look the most animated among the cast.
The application of toon physics isn't just for Luffy but for everything he makes contact with. He's able to turn whatever he touches into rubber. This much is expected, but he can also do unimaginable things like using those rubber surfaces to bounce attacks back at his opponents. He can even use the ground to bounce Kaidō's Blast Breath back at him; there are plenty of things that sound like they would bounce off the rubber, but blast attacks don't sound like they'd be among them.
The cartoon aesthetic is also noticeable when he attacks Kaidō. As he's jerking the Emperor around and swinging him in circles, it's drawn like the in-between frames of a cartoon, right down to the afterimages of Kaidō's eyes. When Luffy finishes knocking him around, he's left wobbling with his tongue hanging out and seeing stars; all of these are common tropes for a roughed-up cartoon character. If Luffy can impose this sort of absurdity even on the King of the Beasts, then there's no telling what else his Devil Fruit will let him do.
If the power of Nika really is cartoon physics, that has so many more implications than simply making things rubber. It depends on just what aspects of cartoon physics are being applied. If the "rules" are anything like Who Framed Roger Rabbit, for example, then Luffy can do just about anything as long as it's for the sake of comedy. Or it could be like The Mask where the feats are ridiculous but the intent doesn't necessarily have to be comedic; this would certainly apply to Luffy when he's bouncing punches off the air to pulverize his enemies. The idea of a cartoon character being able to do "anything" is actually rather vague and can have all sorts of different meanings.
If Luffy's Devil Fruit lets him go full-on cartoon, then he'll really be able to run wild. He could pull safes, pianos, and anvils from out of thin air to drop on people. He could paint a hole on a solid surface and go through it. He could cover impossibly long distances in an instant regardless of obstacles. He might even learn to break the Fourth Wall in good time. If the Fruit's powers really are as limitless as the Five Elders are making them out to be, then the only real limit is Luffy's imagination.
There's also the question of what further changes can be made to One Piece's aesthetic when Luffy uses his Devil Fruit Awakening, especially in the anime. Toei might even incorporate some western animation techniques for Gear Five if only briefly. It wouldn't be surprising to start hearing Hanna-Barbera sound effects in the background. As with the Devil Fruit, it's all about how far they're willing to go.
Despite Chapter 1044's spectacular showcase, it's only the tip of the iceberg. There's still so little that's known about just what Luffy's new powers are and what can be done with them. If Nika's powers really are that of cartoon physics, then there are still plenty of avenues left to explore.