Sanji is the cook of the Straw Hat Pirates and one of the strongest fighters that Luffy has under his wing. He fights with the famous Black Leg Style that he developed under the guidance of Red Leg Zeff. Currently, Sanji has a bounty of 1.032 billion berries on his head that he attained after defeating Queen the Plague of the Beast Pirates.
The fact that he was able to defeat Queen means that Sanji is now one of the elite pirates in the One Piece world. However, there is still room for him to grow. As such, Sanji can still attain several other power-ups in the future of the story and while some of them are minor, others could shoot him to the next level.
5 Internal Destruction Haki
Sanji is a great user of Haki, however, the majority of his powers focus around Diable Jambe and other scientific abilities that he recently awakened in Wano. Sanji possesses great Haki, however, he hasn't advanced this skill to the next level just yet. In the future, if Sanji is to stand a chance against some of the strongest pirates and marines out there, he will certainly need to improve his Haki.
That would automatically make all of his attacks stronger. One of the strongest forms of the Color of Arms Haki is Internal Destruction Ryuo. Luffy learned the skill in the Wano arc and with its powers, he managed to destroy his enemies from the inside without making contact with them. This skill would be great for Sanji to use against enemies who have extremely tough exteriors. Once mastered, this would enhance Sanji's Black Leg Style even further and make him a master of kicks like no one else in the world of One Piece.
4 Rankyaku
When it comes to combat with legs, Sanji is a master like none other. Sanji exclusively fights with his feet in combat because his hands are only for cooking. He has mastered the Black Leg Style over the years and he continues to improve it even today. However, Sanji is still lacking in some ways when it comes to fighting with his legs. Specifically, Sanji cannot fight from a distance with his powers just yet.
One interesting power that could remedy this would be Rankyaku, or Tempest Kick. This is one of the arts of the Rokushiki of the Cipher Pol. It allows them to produce a slicing attack with their legs from a distance. Sanji already knows how to use Skywalk, which is based on the Moonwalk of the CP9. If he learns Tempest Kick as well, he would gain an advantage in battle as he would then be able to attack from a distance as well. Furthermore, he could also combine his Tempest Kicks with Ifrit and Diable Jambe, making them light up on fire.
3 Future Sight
Future Sight is an incredible power that few in the world of One Piece can wield. According to Rayleigh, only a handful of the very strongest possess the ability to see the future. Those who possess the Color of Observation can learn this skill when they train their power to the fullest. Sanji is a master of the Color of Observation and even Luffy has acknowledged this in various instances.
More importantly, those who are especially attuned to the emotions of living beings can develop this form of the Color of Observation Haki. Sanji, as fans know, is a character who revolves around emotions that his mother protected. Thus, he has a good chance of gaining this power. It is almost a certainty that later on in the story, he will gain the ability to see the future as well.
2 Magma
When it comes to the power to burn enemies, magma is one of the most effective things in the world of One Piece. In fact, flames have often been shown to turn into magma when they burn even hotter in the story. This was seen especially in the Wano Country arc, when King's flames turned into magma as they burned hotter. Kaido's flames also turned into magma as the fight against Luffy progressed.
Sanji has the ability to use blue flames, and while technically they should already be burning hotter than magma, perhaps, he can develop this skill as well. It would certainly be useful to him as it would allow him to fight from a distance by propelling magma forward. Whether this would be a direct upgrade to his burning powers is something that only Oda can answer.
1 Conqueror's Haki
Conquerors Haki or the Color of the Supreme King is one of the strongest abilities in the world of One Piece. This is a skill that people are born with and it cannot be learned, unlike the other two forms of Haki. Sanji has not awakened this skill just yet, however, he certainly has the potential to do so. It must be remembered that Sanji is royalty and possesses king's blood, as he is a descendant of the Germa Kingdom.
The people of this Kingdom ruled North Blue 300 years ago as Kings and Sanji is a direct descendant of them. He has shown leadership qualities on various occasions, especially when he led a mission on the Sea Train to save Nico Robin and, later in Dressrosa, when he set out to Zou all by himself. Sanji is a Prince of the Germa Kingdom and it is exactly this facet of his character that can help him unlock the power of the Color of the Supreme King later on in the story.