We all don't know what happened between BB pirates and RA, if anything even happened. What we are certain of is just that Burgess ended up on their HQ island and sent word for reinforcements. We later discovered that the RA changed HQ and whatnot. I think that it may be very possible that BB's crew got to the old RA's HQ too late, the RA already went on their way to Reverie to accomplish whatever they had been planning. What is also possible is that BB found something on that island, perhaps some plans of the RA's or some sort of evidence of something big which led him to persuade something. That SOMETHING might have actually gotten Shank's attention, therefore the need for him to go and inform the Gorosei. All these speculations could only be true if BB was indeed the person Shanks wanted to talk about. Your thoughts?