1. Exploding Kicks (King can both use Flames & Explode so it's fitting, it will be like Superior Version of Ideo's Punches)
2. Black Flames (His CoA alone isn't enough & so are his Flames, so Sanji might learn to Imbue his Flames with CoA, not just his Leg)
3. Unlock Barrier Haki (He will remember Ivanko's Death Wink which is a CoA Move & he will learn to use it with his Kicks)
4. Concentrated Flames (He will realize that he was always releasing too much Fire & never Focus it in one point)
5. Completely New Power (Maybe his New Body will react to repeated use of Diable Jambe/Fire & Unlock another Genetic Ability)
These are all possible imo