I think there have been some major hints that Luffy’s next Gear 4 form will be inspired by sumo wrestling.
- One of the first things that Luffy does when he gets to Wano is fight Urashima, samurai of the Flower Capital and yokozuna-ranked sumo wrestler. Yokozuna is the highest rank in sumo wrestling, like a Champion.
Luffy matches Urashima blow for blow with the power of Gear 3, and defeats him, making Luffy (technically) the current sumo wrestling champion of Wano. (sorry Kawamatsu!)
2) Luffy has another sumo match in Wano, the second time being while he is being held in Udon Prison. This is where Luffy trains to learn advanced Haki, Ryuō, with Hyōgorō. The stakes are much higher in this fight, with Luffy facing death if he falls out of the ring or enslavement to the Beasts Pirates if he loses.
Luffy even ends up facing (a temporarily confused) Big Mom in the sumo ring, and manages to remove the blade collars at the critical moment when she forces them out of it.
However, Luffy hasn’t quite mastered Ryuō at that point and still gets overpowered by Linlin. However, note the open palm strikes Luffy and Big Mom are using.
3) I think Oda has given us another hint about Luffy’s next form at the Wano Fire Festival. Kozuki Oden is remembered there as like a guardian deity of Wano, but he is depicted in their tribute to him not as a swordsman but with an open palm strike, the classic move of sumo wrestling. Even though Oden was a deadly swordsman, he is remembered more as a personification of strength, a trait of sumo wrestling (and Luffy) as well.
4) Luffy is now fighting Kaido and Big Mom at the top of Onigashima, which is now flying like a giant sumo wrestling ring. Oda has raised the stakes again, with everyone in Wano facing extinction if Luffy loses this match. And if any of the Devil Fruit users up there get knocked off the ring and down into the sea, they lose! I believe that Luffy will combine his new Ryuō Haki with a new sumo wrestler Gear 4 in this final fight in Wano for all the Poneglyphs!
*Theory by ricanhavoc