The Yonko are the pirates with most authority on the seas of the One Piece world, second only to the King of Pirates himself. As such, they have a tremendous influence that spans over multiple islands. The Yonko are powerful enough to take islands under their protection – meaning these islands rely on the Yonko to shield them from other pirates or the government. In exchange, they usually have to pay the Yonko back in material goods or man power.
Monkey D. Luffy has now reached new heights as a pirate after Wano's conclusion in the New World, contrasting his debut as a pirate with a small crew in East Blue. He started his journey in a small barrel and has now reached the rank of Yonko after defeating Kaido and his crew. With the said rise of Luffy to power, he has taken territories under his wing, with more to come along as the story progresses.
Fishman Island
The underwater dream island that leads into the New World was the first island that Luffy took under his protection. Indeed, the beautiful island that is home to Mermaids and fish folk was under threat from Hody Jones, a Great White Shark Fishman, and the captain of the New Fishman pirates. Hody was an extremist believer of Arlong's ideals, going as far as murdering the late Queen Otohime out of his pure hatred for humankind.
While strong, Hody was dealt with effortlessly by Luffy and a bigger threat in Big Mom's underlings, Pekoms the lion Mink and Tamago, showed up. This was due to Luffy having eaten all of her candy, which the inhabitants used as payment to keep her at bay as the island was under her control. Originally, the island was under Yonko Whitebeard's protection, however, following the events of Marineford, Whitebeard perished during the war to rescue Ace and Big Mom took over. Luffy, with his classic recklessness, defied her and declared war against her, meaning that he has pledged to protect the island from her tyranny.
Since the Fishmen are a powerful race of underwater warriors, Fishman Island harbors a lot of powerful characters, most notably the Sun Pirates, who were once led by the former Warlord and current Straw Hat, Jinbe. The most important asset that this island offers, however, is an Ancient Weapon, Poseidon. Princess Shirahoshi was revealed during the Fishman Island arc to be the Ancient Weapon that can destroy the world by controlling all the Sea Kings, making her possibly the most powerful character in One Piece who will be incredibly important during the final phase of the series.
Wano Country
Wano is a country of many powerful Samurai and has been the center of the recent Four Emperors Saga that has just concluded with the defeat of Yonkos Big Mom and Kaido. Although filled with strong samurai, the island was under the control of Kaido for over 20 years. It has now been freed by the Straw Hat Pirates and their allies after a very long wait, and following their freedom, the country has pledged to assist Luffy in his upcoming trials in becoming King of the Pirates. Being the home country of many powerful warriors, such as the remnants of the 9 Red Scabbards like Kin'emon, OKiku, Raizo, and Denjiro, among others, the country is nearly impenetrable.
All the supporters of Oden have now united under the new Shogun Momonksuke, who himself can now turn into a gigantic dragon akin to Kaido. Most importantly, the Oni Princess Yamato, who is also Kaido's child, is also a guardian of the land and is protecting the land in Luffy's stead. With the flag of the Straw Hat Pirates flying high over the country, Wano is an incredibly powerful asset to Luffy and his crew. This island also holds another Ancient Weapon, Pluton, which should come into play later in the story.
Potential Territories
As revealed by the series author, Eiichiro Oda, One Piece will be entering the final saga with Wano's conclusion, which means Luffy will continue to grow in power quicker and acquire new territories following the fall of two Yonko. One such island could well be Sphinx Island, Whitebeard's home. Since he has already died, his home place is in need of protection and Marco cannot keep it safe on his own forever. Furthermore, Whitebeard was a father figure to Ace, Luffy's brother, and could this come into play here. Another possible inclusion could be Amazon Lily. With the dissolution of the Seven Warlords of the Sea, the island has been left more vulnerable to threats such as the Navy, even though it is already protected by Hancock's very powerful Kuja Pirate crew.
Other already visited islands that could go under Luffy's territory in due time could very well be the islands of Alabasta and Dressrosa. The crew spent a lot of time in these countries and freed them from the grip of two Warlords in Crocodile and Doflamingo. As of now, these two islands are aligned with the World Government, however, with the unstablity in the One Piece world after Reverie and Wano, it's very well possible that they could align with the new Yonko, Monkey D. Luffy as well.