God Valley is definitely significant but it isn't raftel. To start off, we already know the island is called Raftel and not God Valley as stated by Robin. The island has already been visited by Big Mom, Kaido, WB, Garp and Roger. If that island was Raftel why have these people not returned there? Roger later went on a final voyage and reached raftel. Would be wierd if Roger already went to the island before his grand voyage. Then the celestial dragons are involed in god valley. If the one piece treasure was at god valley, the celestial dragons would have taken it already.