We all saw how Luffy used haki and gear second at the same time in order to use the red hawk against Hody. If he could do that with one of his arms he could possibly cover his entire body with haki while he's in gear second. He would then be covered with an aura of fire making his attacks more powerful and giving him the slight of a super saiyan. If he tried that he would be major trouble for any logia he faces in the new world but would it be enough against Akainu?