Luffy, an Emporer? Nah. He's reaching for greater heights than that! So for now, we'll just have to settle for him being a supernova.
Speaking of the supernovas, judging by how shocked most of them were about the news of Ace's execution, most of them may high-tail it out of Sabaody sooner than expected. I kinda doubt that the news will affect guys like Kidd or Law, though (especially Kidd, considering his opinion of the World Nobles in chap. 500).
Oh, and I have a feeling that the Rayleigh situation will be resolved long before Garp gets the chance to do something about it...