Powers & Abilities Luffy should have Three Forms.

[Dettles]

Zoans have Three forms. A Human form, Animal Form, and Hybrid Form. Right now Luffy is missing a Form. Either a Full "Nika" Form or More likely a Hybrid "Nika/Luffy".

 

[Yoho]

I feel G4 is the hybrid form

 

[moreha9685]

G5 is the hybrid.

 

[Dr.ShiNoGekai]

Form 3 would presumably have to be a future evolution of Nika. Gear 4 is a technique/form within Luffy's existing rubber + haki toolkit, while Gear 5 fundamentally comes from awakening the DF. The interesting question is what an awakened fruit could evolve into from here