I ultimately think it happens at laughtale, with buggy always falling upwards i can't see oda not having him(crossguild) there especially since buggy said he just needed to reach it first. At that moment zoro defeats mihawk making zoro the Wss and luffy the Pk as the Pk needs the Wss.
I do think the encounter with the wg will come sooner rather than later. Oda established that The moment vivi is captured Luffy,Zoro and the crew pulls up to rescue her i don't think this happens after laughtale etc.