The thing about it is I want a powerup that has its own uses, but doesn't take over his other powerups. The amazing thing about gears is Oda was able to make two distinct power ups that are totally separate from one another and have their own distinct uses. I'm afraid that if Luffy adds another gear, that gear will make either or both gears obsolete. I love how Oda's been able to make it so gear 3 isn't just an upgrade to gear 2 where Luffy won't really need gear 2 anymore. Gear 3 has its own uses that will help Luffy in situations where gear 2 won't work or doesn't make sense, and gear 2 has situations where gear 3 won't work or doesn't make sense. I'm wondering how Oda can make another powerup that is cool, distinct, but at the same time doesn't give Luffy a reason to not use his other two.
With haki, I'm worried his haki will become so strong to the point he won't need gears, or even worse, to use his devil fruit at all. That's one of the big problems I have with haki as an ability in itself, it's way too overpowered as a concept. So far, Oda hasn't shown any real weaknesses or side effects of haki to the point where it seems like if Luffy mastered haki, he wouldn't even need his DF anymore. I mean, if you really think about it, someone with a complete and total mastery of the 3 hakis would become invincible. They can be completely invincible and invulnerable to any and all attacks with their armament haki and predict everything an opponent is about to do. And because their haki is so strong, they'd be completely immune to DFs since their haki basically overpowers the opponents DF, that is if my understanding of how it works is correct. And that doesn't even include conqueror's haki, which can pretty much neutralize everybody and anybody if your haki is strong enough. Plus, you don't lose your ability to swim. Unless there's some serious side effect we don't know about, haki as a concept is so incredibly broken. If Luffy wanted to go into a god mode, he'd just need to master all his haki.
As for the OT, as much as I want a god mode Luffy, I don't think it'll happen. I think he'll refine his powers by EoS, but he won't be pwning anybody and everybody he desires. I think it might become an interesting idea for Luffy to lose his DF to BB, have to relearn how to fight, but basically become a god in the process by becoming the world's most powerful haki user and, as a result, invincible. But when I think about it that way, it would've made more sense for Luffy to have been born with immense haki in a world of devil fruits and have to develop himself to the point of having godlike powers. When I look at it that way, I just don't know what to think lol