In my opinion, 3D2Y should have went this way: Luffy is struggling to master and control his Haki because the guilt from Ace dying, plus the paranoia of not being able to protect his friends because of this guilt, is weakening his willpower, which Haki is literally the manifestation of. Then, Luffy in normal mode being beaten up by World because World is too strong and too fast. Luffy comes back in their second confrontation, and punches him from the side with Gear 3, which proves to be effective, Gear 3 gave him the strength to harm World, but then World dodges the next Gear 3 strikes because he's too fast. Luffy uses Gear 2 to edge out the speed, he's faster in Gear 2 and can now hit World, but his strikes aren't strong enough, like in Gear 3, to be super effective. The dilenma is Luffy has to either pick speed and be able to hit World with blows that aren't strong enough, or enough power to harm World but the strikes aren't fast enough. He can't stack the Gears because his body is still recovering from his training and Marineford, plus his previous encounter with World, so stacking the Gear would do more harm than good to his body. When World slams him into the wall, Luffy thinks of his Nakama and Ace. Hearing World talk badly about his friends clears Luffy's resolve, Luffy knows he cares about his friends and is better than World and is better than what he's giving himself credit for, clearing up his guilt and paranoia and allowing him to control his Armament Haki. Now that he can control his Haki, he uses it in conjecture with Gear 2 to have BOTH the speed and power to hit World. This would have:
1. Given Luffy character development
2. Avoided the Fairy Tail-esque BS power-up to pull a victory
1. Given Luffy character development
2. Avoided the Fairy Tail-esque BS power-up to pull a victory