Luffy has always been rather unpredictable when it comes to his fighting style. About two years ago, he surprised all of us with the reveal of Gear 4th, a bouncy, kabuki-inspired behemoth. Now he continues to surprise us with a new form for Gear 4th, Snake Man.
When I first heard that name, I didn't know what to think. After all, all of Luffy's Gear 4th abilities up to this point have been about blowing himself up like an oval balloon, puffing outward and rounding himself out.
How could that possibly play into anything even remotely snake-like?
Then it dawned on me: there's more than one kind of balloon. In fact, there's one type of balloon that's specifically snake-like: the pencil balloon!
Pencil balloons are the basis of balloon modelling, and in their base form are the prototypical balloon animal, the balloon snake. With that in mind, it seems fairly clear that when Luffy activates Snake Man, his body or his limbs, rather than inflating along their width...
...will inflate along their length. The panel where Luffy is shown activating Snake Man even suggests that this may be the case.
Unlike in his activation of Bounce Man, Luffy's arm does not seem to have become wider, despite the fact that he has clearly already put the air into his arm. Since all we can see here is the middle of his arm, it seems likely that his arm has extended length-wise off-screen in this panel.
This concept is not without precedent, either. Luffy has already demonstrated the ability to will air into his limbs so that they are long rather than wide in one of Bounce Man's techniques, Gum Gum Culverin.
As Doflamingo pointed out, Luffy was able to manipulate and lengthen his outstretched arm without drawing it back in to tighten it first. This is just conjecture, but it may be that Luffy is controlling the air within himself to stretch his arm further. We already know that Luffy is capable of doing a similar feat thanks to his mastery of Gear 3rd.
Originally when Luffy would use Gear 3rd, the air in his limbs would taper off the further from his hands or feet it got...
...but after his training with Rayleigh, he gained the ability to force all of the air into one spot, allowing for denser and stronger attacks.
Having demonstrated this level of control, Luffy is very likely capable of doing the exact opposite and elongating his limbs rather than expanding them. As it happens, he's even done that in the past as well, and to great effect.
At this point it's probably common knowledge that Luffy's Gear 4th forms seem to be based off of his improvisational forms from the past, such as Bounce Man having the muscular upper body of Nightmare Luffy...
...and Tank Man having the rotund belly and tiny legs of Water Luffy.
Because of this, the common consensus was that the next form of Gear 4th would likely be reminiscent of Afro Luffy, but with the name Snake Man, it's hard to see how that aesthetic would play into it.
Like I said, we're looking for something long, and other than Gum Gum Flail, there was nothing long about Afro Luffy. Up to this point, there has only been one time where Luffy has been able to make his limbs long and remain long without having anything as an anchor: Gum Gum Octopus, as seen in his fight against Enel.
In this form, Luffy releases the tension from his limbs, allowing them to become long and be swung about more freely. With this, Luffy was able to randomly bounce his limbs off of Enel's ship, attacking him without any specific will and overcoming Enel's Observation Haki, then known as Mantra.
Similarly, Snake Man will likely have elongated limbs that do not retract and can be swung about freely, but more on that later. Before I go too far into applications, I would like to make one other point about the aesthetics.
Please refer back to the earlier pictures of Bounce and Tank Man. Take note of where Luffy's air has been allocated. For Bounce Man, it is all in his chest and arms, and for Tank Man it is all in his torso. In neither form is the air allocated to his legs. Because of this, there were theories that the next Gear 4th form would involve inflating Luffy's legs, often citing the wax armor created by Mr. 3 for the possible aesthetic inspiration.
This may still be the case, as the armor does make Luffy's limbs longer, but the bulky design defeats the purpose of elongating the limbs in the first place, free and flexible movement. With that in mind, I am quite confident that Snake Man will take more after Octopus Luffy than Wax Luffy. It could also stretch out his back, though, making his body as a whole longer, but the main focus would be his limbs.
Now let's move on to the applications.
First, as an enhanced version of Gum Gum Octopus, it could potentially be used similarly to Octopus, moving erratically enough to carry no will and overcome Katakuri's future sight in the same way that Octopus overcame Enel's Mantra. Luffy could either do rebound attacks or use each limb to attack from various angles, confusing Katakuri's Observation Haki.
Second, since the other two forms of Gear 4th have both revolved around tension (bouncing off of things and bouncing things off of Luffy), it seems likely that Snake Man will either continue that trend and make Luffy's limbs bouncy for rebound attacks as said above or break the trend and focus on a complete lack of tension, allowing his limbs to seemingly stretch into infinity. If the latter, Luffy could do techniques similar to Katakuri, who can extend his limbs and fight from a distance without needing to retract his limbs due to tension.
As for actual techniques, it could be used for various wrapping or binding attacks. We already know that Luffy is capable of wrapping his body around targets, whether it's his arms...
...or even his neck...
...so with elongated limbs, these wrapping techniques would be even easier, as he could execute them much more quickly and wouldn't have to worry about his limbs rebounding if the target escapes.
Since it is called Snake Man and he presumably created this form while on Rusukaina, he may have drawn inspiration from his fight with the Boa sisters, who used their snake Zoan forms for various snake-based techniques such as constriction...
The last one could be especially helpful in the fight with Katakuri given how much difficulty Luffy has had with dodging Katakuri's attacks.
My final and favorite idea for the application of Snake Man comes from my original conception of it, the pencil balloon. As I said before, the main purpose of pencil balloons is modelling, as their shape lends very well to bending, twisting, pinching and various other sculpting techniques which leads to near infinite shape possibilities. I sadly cannot think of too many myself, but Luffy could potentially use this ability to shape his limbs into various constructs, such as a balloon sword.
On its own, the balloon sword may be much the same as Luffy's regular arm, just shaped differently, but knowing Luffy and his habit of copying techniques off of who he's fighting, he might just be able to sharpen it in much the same way that Katakuri turned his fists into blocks.
It's still unknown if Katakuri did that using Haki or if he just has that high of a level of control over his mochi, but even if it's the latter, the Mochi Mochi Fruit is similar enough to the Gum Gum Fruit that it wouldn't be at all surprising if Luffy could do that too.
We also know that Luffy is dexterous enough to tie knots quickly, as he was able to tie Sandersonia and Marigold's tails together in the heat of battle, so there's no reason he wouldn't be able to make balloon shapes out of his own arms in battle too.
I'm not actually too confident that balloon modelling is the direction Oda will take with this, but I think it would be really interesting and would work well with the rest of Snake Man's possible applications.
Fortunately, we probably won't be waiting too much longer to find out the answer to this mystery, so hopefully I was at least a little close.
Thank you all for reading,