Sanji hates being a Vinsmoke but Oda has made it quite clear that despite Sanji hating his origins, he is also tied to the Vinsmoke Lineage.
Sanji has received a Raid Suit which only the Germa wear, his last name has been changed to Vinsmoke on his bounty poster and he is essentially recognized as a prince of the nation by both World Government, Germa and even Ivankov who reminisced that Sanji didn’t tell him that he was a prince.
So for all intents and purposes Oda is highlighting the fact that Sanji is a Vinsmoke despite his hesitance the world and Oda see him as part of that family.
Now here are my reasons for why Sanji will be King of Germa!
-This was someone else’s fan theory that I thought was excellent. Because of the genetic modifications the Vinsmoke children are sterile(can’t reproduce). This would mean that Judge’s family would die out. However what if Judge had a child who wasn’t modified and who can pass on the Germa lineage. Enter Vinsmoke Sanji! I think this would also validate that Sora didn’t want this for her children and that is why she tried so hard in addition to not making them monsters to stop Judge.
– Sanji once said that he would be the most troublesome man for the Navy back on Thriller Bark to Kuma. Now we know that as a fighter Luffy and Zoro will be above him in terms of threat based on combat, however what if this was alluding to the notion that Sanji would lead the mythical Germa army, who are conveniently portrayed as the most evil enemies to the marines. i.e Sora Warrior of the Sea.
– It is no coincidence that Germa doesn’t have any land and is a sea faring Kingdom and Sanji is all set to find a mythical sea that hasn’t been found and thus hasn’t been occupied. I think that once Sanji finds the All Blue, Germa will relocate to that place finally having a place to call home. Judge was extremely upset that they had no home to call home but what if they found a new home at the All Blue.
– Sanji utterly changed the hearts of his family when they saved him at the cost of their own lives. Whilst Judge did give that speech about Sanji I think it was more the fact to highlight how wrong Judge was regarding Sanji rather than Sanji being an actual failure. Sanji is going to be the strongest Vinsmoke (if not already) and the Vinsmokes respect strength. Sanji being admiral level at the end of the series and leading Germa will highlight to Judge that his greatest failure has become his greatest strength and that his mother was right!
– Reiju herself said that she was glad that the Vinsmoke lineage wasn’t completely devoid of conscience and when she looked at Sanji she was happy that their blood line had good. Sanji is still included in Germa’s bloodline and what better way to revolutionize Germa than to be led by a kind hearted man. If Sanji became King of Germa he would revolutionise it into a peacekeeping force that helps war torn countries and distributes food.
– This is speculation, but I feel that Franky is going to take over Germa’s scientific laboratory. When Sanji becomes King, I think he is going to abolish cloning soldiers and instead have Franky use the science of Germa for good. I mean Judge was a peer to Vegapunk and I think that Franky would benefit a lot by having access to their technology.