To become the strongest, Luffy will doubtlessly continue to evolve. Gear 2nd and 3rd would not guarantee his victories in the future, especially in the New World. There is little doubt that in the future, Luffy shall develop a fourth gear.
So, what's the fourth gear consisted of? And what will be the fourth gear's side effect? What price will Luffy pay in order to surpass his own limitation? Keep it in mind that so far Gear 2nd and 3rd are theoretically plausible from a scientific perspective.