Luffy’s creativity with his devil fruit certainly knows no bounds, however, I have 3 ideas, two of which could actually form the basis of an entirely different fighting style like a Gear Luffy might use in the future.
I, personally, think Luffy will have a total of 6 Gears (like a transmission) by the end of the series, 6 of course being the fastest and strongest.
1-Luffy can harden his body with Haki to something much harder than solid steel, and by using the same method as Gear 4 of drawing parts of himself inward and using the elastic force to shoot them out incredibly quickly, Luffy could use parts of himself like his fingers or potentially entire limbs, pulling them in and releasing them like bullets.
2-Luffy often draws on his opponents abilities to influence his own attacks. Luffy has now seen Bellamy in action twice, and Luffy essentially has the better version of Bellamy’s fruit. Gear 5 may be his adaptation of using Bellamy’s Spring-style combat. He can use his joints, feet, and hands as springs simply by twisting and compressing them (think of his elbow and wrist as springs and what kind of damage he can do already with normal punches).
3-Vulcanized rubber is a thing, and Luffy can already compress and harden parts of his body to extremes. I think Luffy will have to make a very serious and deadly attack style to deal with some of his future opponents. So, I think Gear 5 or 6 will involve Luffy compressing his body to incredible degrees and hardening that further with Haki to become nearly invulnerable, then causing extreme damage by gripping/wrapping/placing his hand on an enemy and releasing that energy like a one inch punch.
Just to be clear, Awakening and better use of Haki/Advanced Haki does not qualify as a “Gear” which is a transformation of his body using his normal Paramecia powers.
*Theory by Backwardslotus