I realised today when i was watching One Piece episode 145 that when Captain Buggy was trying to find Captain John’s treasure he found himself on a unknown island. This island has a skull just like Onigashima.
Soon he realised that the island is wrong . I think Captain Buggy was actually trying to find Onigashima.
As we know, Captain John was a legendary pirate in Rocks Pirates alongside with Kaido so there is a possibility that he hid his treasure in Onigashima where Kaido lives.
Captain Buggy says that treasure have monsters as their guardians. So maybe Captain John hid the treasure on Onigashima so only strong people will be able to take it from the island. There are more common points: treasure are always gold and diamonds, and hundreds of years ago, Wano(where Onigashima is) was known as the Country of Gold.
Another thing: Buggy was trying to find treasure in underground caves and Onigsahima also have an underground floor(as stated by Yamato) that is not used anymore. If there is really Captain John’s treasure on the island then it would pe pretty useful after the war to restore Wano to its rightfull place.
*Theory by ShivSharma5