I'm not talking a "Water 7 to Enies Lobby" scenario (except for when they get to Laugh Tale). I mean it'll take place at Beehive, Mariejois, Marine Headquarters, and more at the same time. If it had to have a centralized place, it'd be Lodestar. It could be where the Straw Hats initially gather until Kuma warps them to other islands for their respective mission and where all forces would eventually gather for this giant war that Oda has said makes Marineford pale in comparison.