The way I see it, being a Yonkou is part of becomming Pirate King.
I'm not sure if this is true or not, but the picture I got, was there were no Yonkous when Roger was PK. And neither was there before.
Roger simply conquered the whole New World or atleast the most of it. The yonkou are pirates that have "shared" those parts of the New World after his dead. :-)
Can't see this happening. SH were always facing some really strong enemies that they couldn't defeat right off. I believe Yonkou means - one of the four strongest pirates. For storyline it's important that there is enough space for characters to develop. And if Luffy becomes Yonkou who will he fight with?
Concept of yonkou has been defined as the 4 strongest pirates... and to balance that exists 3 Admirals and 7 shichibukai ...
When Shanks met WB .. there was a vibe that 4 yonkous also exist never to cross each others path... and WB having territories simply meant that island is under his protection and anyone raising finger on that has to fight WB... but in case of Big mama .. it seems that islands are under her jurisdiction and she pretty much exploits islands ..
Also we have not seen Shanks owning any territories .. Also someone pointed out that luffy defeating a yonkou with help does not mean he will be considered as yonkou.. from what i remember .. BB killed WB when his half face was blown and he was all alone ... and they pretty much ganged on him... BB became yonkou.. did he become that later .. or just after the war.. he didn’t look like a yonkou when he ganged on Bonney for that matters.. so how he became one is also pretty much unknown.. but the fact that he killed WB and owns his DF... definitely adds to him becoming a Yonkou..
Also ... Just for the sake of maintain peace it seems that yonkous do not cross each others path.. but if luffy is there and he sees something bad happening he will not care about that... and thats what he will fight for.. against other yonkous .. against WG... against alliances...
And being a PK later for luffy means that when he says this is like this in the sea, for the islands and may be take a stand against government at certain point of time.. no pirate will go against it... Luffy will have the free will... May be pirates will be there who will not support him.. but they will also not oppose him is what i meant...
Roger didn't conquer shit
Roger never cared about territory
Roger was simply " Free ", free from anything, nothing could tell him what to do and what not, but in order for him to reach that level, he had to face all those who wanted to stop his ambition, and after many epic fights and adventures, he reached the top which allowed him to finally become " Free "
Roger's whole life turned about the idea of freedom, if I have to give Roger any description, I wouldn't say he's a revolutionary, not like Dragon, what he does is just weakning the WG in a careful way, something that Roger lacks, he wasn't careful at all, he faced whatever danger ahead of him. Roger had ideals, and had strength too, and his death gave birth to something that Dragon cannot pull (from what've seen from his so far)
I think in order to understand Roger, we need to go back to the very core of OP : Dreams, freedom, and Oda in Luffy's flashbacks reminded us very well of those two concept, especially when Ace was talking with Luffy at that cliff after Sabo's death (I don't believe he's dead btw)
I'm sure Roger was considered to be a Yonkou at some point in his career, although he probably never wanted or formally accepted it for himself. Luffy will probably follow the same path/ideal. Being a Yonkou isn't like being a Schibukai where you can "apply" to get the position (Law sending 100 marines, the guy who vouched for Blackbeard @ the first Schibukai Meeting that we saw).
Go crackpot prediction:
The way I see things, the whole Yonkou system needs to collapse before Luffy can become pirate king. Not really that hard to do too... Remnants of Whitebeard's crew support Luffy, they're about to (likely) destroy Big Mam with Law's help, and I'm convinced Shanks' role is to unlock the path to Raftel for the Strawhats and/or cripple Kaidou in the process. With the Yonkou system then destroyed, it comes down to the Supernovas, which is when they call a great pirate meeting (that accomplishes nothing but shows heavy support for Luffy) where they agree that whoever gets to Raftel/finds One Piece first shall be known as the Pirate King. Guess who wins.
Honestly there are so many roads Oda can take that would be quite interesting which just goes to show you how good he is. I don't feel like he is stretching his series like Naruto/Bleach has done. I only bring this up because it goes to show you One Piece will likely be around for years to come.