That panel of Luffy is ominous. Luffy does sometimes smile in a devilish fashion, but usually only when he gets a leg up on someone, not while he lies unconscious, seemingly. If this is intended to be Luffy's awakening, and with the seeming return of Joyboy, could we see a sort personality shift for a bit? Oda did say to keep an eye on Luffy. Maybe there's more to this than just some power up.