The Cayo Perico Heist DLC is scheduled to release on 15th December, and simply put, it is shaping up to be a massive update to GTA Online. Rockstar Games, ahead of the announcement, made it clear that this would be a significant update, going so far as to call it the "biggest ever update" for the game.
That kind of talk piqued the fan's interest to no end, and expectations were through the roof for what Rockstar could be planning. Recently, the veil was lifted, and players got a first peek at what awaits them in GTA Online come mid-December.
The newest location in the game, Cayo Perico, an island which has essentially been turned into a drug lord's private compound, will serve as the location for the newest heist in-game.
As to where this island could be on the map, Rockstar is still to reveal it, though some theories are popping up on the Internet (of course there are).
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GTA Online Cayo Perico Heist: Where will the new location likely be on the GTA map?
Fast travel through Submarine HQ
Many have speculated that the island would not be on the map, and the Submarine HQ would act as a fast travel-like mechanic to travel near the island. This could be the case in reality, as in the Rockstar Newswire post, the publisher mentioned that players would be returning to Los Santos after the heist.
It could signify that the islands are not close to GTA Online's Los Santos and are maybe even in another country. It would be safe to assume that Cayo Perico is in no way part of Los Santos as it essentially is a home for a massive drug cartel.
Zancudo
Another popular theory backed by concrete evidence suggests that Cayo Perico is a Pacific Island located on Zancudo. This would indicate the island being just off the coast of Los Santos, thus allowing players to travel to and from the location easily.
The primary evidence pointing to this seems to be the co-ordinates, which would put the site right along the Zancudo River. While it isn't part of Los Santos in all likelihood, it could be an island just a few miles off-coast.