I am always more convinced that Luffy will have his new and final Gear Fourth form and it will be called “GEAR FOURTH: TIGERMAN”
This is based on only 2 reasons:
1-All Luffy’s Gear Fourth forms are inspired from and created to counter the animals in the island of Rusukaina. There is also a tiger in Rusukaina.
For a comprehensive suite of abilities to take out enemies on Rusukaina, Luffy would have needed to develop techniques to deal with flying monsters (i.e. birds) and hence the flying form ‘Boundman’, defensive monsters (i.e. tortoises) and hence the form ‘Tankman’ , quick monsters (i.e. snakes) and hence the form ‘Snakeman’, and deadly (i.e. tigers) and hence the form ‘Tigerman’ . He might even learn a thing or two from these opponents.
2-In ancient Japanese mythology, the Tiger and the Dragon are considered to be equals. Kaido is a dragon and hence it would be fitting if Luffy were to defeat the Dragon as a Tiger.
I think Gear 4 TIGERMAN will be focused on mastering Advanced Armament Haki like Gear 4 SNAKEMAN is focused on mastering Advanced Observation Haki.
*Theory by letsgomina