Pizarro merging with Red Line could make a ton of sense narratively because destroying Red Line might be the key to unite the seas and create All Blue. Another point in Sanji vs Pizarro that actually has narrative significance. I can totally picture a giant version of Pizarro made of red rock with trees on his head, with Sanji having to overcome his (relative) weakness, which is AP, to destroy Red Line, defeat Pizarro and accomplish his dream.
Certainly makes more sense than Sanji fighting a guy just because he can fly or create illusions.