Final arc theory
I have a theory of the one piece / final arc.
So as they make their way to the final island laugh tale, they are met with someone or something, most probably another member of the joy boy crew, guarding the final treasure. The straw hats battle it out with that thing/ that person. In the end, the entirety of luffy's crew dies. Seeing the bodies of his crew lying about, Luffy goes into a new transformation, gear 6 or gear 7 ( depending whether we get more gears in between)
This new gear gives him the ability to:
1- call onto more gods to help him fight, eg: god of life, god of water, god of lightning( enel) etc. these gods will help him fight any foe. Or he can summon anyone from his past endeavours to help him. He would get immense haki, uncontested fighting power and vegapunk level iq.
2- give him complete freedom. No restrictions regarding physics, biology, etc. he becomes a true god who is all powerful, easily manipulating the enemy into a dog or a flower or smth. He can fly, make his body into weird shapes, change his surroundings into anything he can think of.( I feel gear 5 still doesn't give him complete freedom and has some limitations.) This gear would give him the ability to do anything he can imagen. No restrictions whatsoever.
This theory also goes with what oda said about the last arc being a dark/ devastating arc.