I think Sanji has a legitimate argument to be a Conqueror:
- 1-Sanji is clearly a natural born leader. His crewmates aside from Luffy and Zoro naturally band under him and Groups are drawn to him and make themselves subordinate to him even when he doesn’t want it. Examples: flying fish riders refer to him as master and the G-5 marines (pirates natural enemies) followed him around even when he didn’t want them to and directly followed his orders. The reason I think this is particularly important is because the only other characters in the story that have gained followers that they initially rejected have been Conquerors. Oden with the Scabbards, Zoro with Jonny, Yasuke, Yasuie, Luffy with the Grand Fleet, etc…
- 2-Sanji has on multiple occasions completely turned enemies to allies: G-5, Viola, Pudding, Kinemon. Mr.2, Gin, etc… and the latter was even completely ready to give up his life for Sanji out of sheer respect. The ability to command loyalty completely based on the content of your character seems to be a quality that has closely been linked with Conquerors in the story and has been most prominent with his captain Luffy.
- 3-Sanji definitely has a ridiculous level of willpower. He has some of the most insane endurance feats to date in the series and is easily among the most stubborn characters One Piece has. When his shadow was put in a zombie by Moria his zombie was completely insubordinate and was much stronger than the other zombies due to its weak body due to Sanji’s sheer willpower. He also has extremely powerful and unwavering sense of character as even the physical and psychological abuse he experienced from his family as a child could not change his innate personality or his values.
- 4-Sanji comes from a royal bloodline that literally once conquered the entire North Blue. Several confirmed Conquerors have some kind of connection to royal positions and now Germa on top of that is a family of failed conquerors that lost their control. Judge designed his kids to be the perfect vessels to conquer and he considered Sanji a failure because of his personality. It would be the perfect irony and very Oda like story telling to make it clear that Sanji was the true conqueror all along due to the characteristics that his family considered him a failure for. The whole character arc with him not accepting his name and the raidsuit will probably end with some sort of concrete payoff in the form of a new ability. It would make sense if it were Conquerors’s Haki. Oda specifically had Sanji compliment Momonosuke on the way he claimed his name and it definitely seems that Oda has plans for Sanji concerning accepting his heritage.
- 5-Sanji is definitely going to be a top tier fighter at the end of the series. Conquerors often reach positions of higher status or regard and we now know that he will eventually be widely regarded as one of the Wings of the Pirate King. Strength is also a factor. No matter what anybody says that is clearly a title of stature (one he will share with the world’s greatest swordsman) that is largely strength based. You can claim that the dynamic between Zoro and Sanji is just a character thing but you cannot claim the same for the Monster Trio as a group. The three of them have almost always entirely been grouped together when there is combat involved or to highlight their relative strength. Zoro and Sanji being Luffy’s wings mean that Sanji will have to have the ability to fight with the strongest on the seas. He will probably need Conqueror’s Haki to do that and would be much easier to write if he had it now that we know how it works.
My last points are addressing certain reasons why some people think it is impossible. One I hear often is that Conquerors need to have the drive to reach the top of the world or have strength or status based goals but this is clearly not true and there are several counter examples for this. While both Whitebeard and Raleigh reached the top of the world, Whitebeard’s dream was to have a family and Rayleigh did not seem to have any ambitious goals he seemed to be very laid back from his introduction enjoying a good time and flirting with young ladies.
The next point I hear against it is the fact that it would make Conqueror’s Haki less special if more members of the crew have it but I think that is a fairly weak point as well. We know Roger’s crew had a minimum of four members with the potential to be king at once and Don Chinjao clearly said that when you get to the stronger parts of the world there are countless souls with the quality of king and the pirate king should stand above them. Honestly that’s even more reason for the top members of Luffy’s crew to have it in my opinion.
*Theory by Prince Kronos / Choice_Till_5524