He might never be yonkou 'level' and I would be okay with that. Of course he will become pirate king but you don't really need to be at the very top of the food chain for that.
In fact I would hate it if Luffy would be the very very strongest because that would be too cliche shounen main character.
It would be cool if there is always a little threat that there are a few absolute monsters out there that control the world like Kaidou, Shanks, Big Mom and Whitebeard. Freaks of nature that just can't be measured against 'normal' people.
Luffy already evaded having to face Whitebeard, and he doesn't need to worry about Shanks. My other theory is that he won't fight Big Mom and Kaidou himself but he will have a plan in place to disable them in another way without a fist fight, just like Oda said in the interview.
By then only Blackbeard remains and I feel like he pretty much stole the yonkou title and doesn't really deserve his place up there with the others and Luffy will probably face him in a real fight near the end of the series. By beating him he might take the title of yonkou or in Luffy fashion don't care about it. But I think his biggest strength as a yonkou will be because everybody fights with him and not his monstrous solo strength.
But to answer the question, maybe some Gear 4th variation. I really can't imagine any more gears now or it would be stupid. Awakining and Haki can come into play more. Conquerors haki has to have another trick to it besides knocking out fodder.
