Many people have mentioned the parallels between the new movies and the the "Original Trilogy." Here is why Fire would make sense... hold on to your seats because it is obvious and we have seen it in RTOJ. Fire is Vaders last step. George said "Fire" twice. It would make sense that the same moment that Vader is turned, the first time, is the segway to his dark side. The same as Vader being returned to Anakin as Luke burns his body at the end or RTOJ. The circle is now complete. I have you now!!!