Maybe it's too early for G5 but he could be bringing out a new G4 form.
If Boundman is Luffy's "little bit of everything" G4 form. Then, as morj and many have said before, we may get the extreme variances for the 4 different highlighted benefits that come with G4, before G5 final fight w/ Kaido.
Snakeman - Extreme variance for stretching
Tankman - Extreme variance for durability
We could be getting the extreme variance of the "power/tensile" force form this chapter.
- Luffy's learned advCoA and advCoC, both of which add a deal more power into your attack.
- He could use adv CoA to create attacks like Rayleigh did here.