YES THIS IS MORE OVER ANALYSIS IF YOU DON'T LIKE IT GET OUT
Lets start off with what sanji can do before we get to what he can't.
- He is immune to fire.
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Since his introduction the guy can run through flames where even luffy can't
And he looks bad ass when He does it
You must be registered for see images - He is faster than 99% of One piece
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In case you missed it he flew from dress rosa (One Island) to Green bit (Another island) Before Mingo could hit The ship. - He can Fly
You must be registered for see images - He thrives in close combat
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F!@# your face
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F!@# your face
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F!@# your face
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F!@# your face - He is very capable of Igniting himself
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Guess what? F!@# your FLAMING face
I am gonna kick you in the head
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Just kidding, f!@# your face
- Walls
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Sanji couldn't break it
Its too hard to kick.
It doesn't have a face either.
You all know this but here is the breakdown.
Luffy's fighting style is raw power and destruction. He fights unpredictably and like a tornado.
Zoro fights like a tiger powerful and dangerous. He cuts all the things in his way.
BUT sanji doesn't have Luffy's ability to destroy almost anything or zoro's ability to cut. He is limited to external blows in combat meaning if he can't break through your "walls" he is S!@# out of luck. Sanji has to hit you where you are weak to get his blows to damage you shown as most of his kicks are for your face and your neck. He has no power to harm you internally and if you are like a wall he can't touch you. His worst opponent Is Diamond Jozu Regardless of admirals or Yonko's if Sanji can't break through your surface he can't hurt you making the man made of diamond his worst nightmares.
AND THATS WHERE HIS IDEAL POWER UP COMES IN!Your Poo Poo brain said:But tomato! We were promised power ups!
Foreshadowed fire power for sanji.
Diable Jamble
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Here sanji explodes in rage. Something that may have been comical but its very important. The perfect power up for sanji would be Rokuogan. Ill give you a second to go google that. ♫♫♪Jeopardy music♪♪♫
Its the technique Lucci used against luffy where he transferred force inside of his body. Sanji has always been foreshadowed to use fire to burn his enemies. He uses Rokushiki to fly so he can already use Rokushiki.
So if Sanji as portrayed is to follow the third foreshadow flame attack it would make sense for Him to use Rokugan in tandem with Explosive force to destroy the enemy from the inside out. It would overcome all his weaknesses (Doors) and complement his strength, as he doesn't need to be long range, thriving in close combat and able to close island levels of gaps.
TL;DR
Rokugan + Explosive legs to damage the enemy from the inside out making them explode on the inside.