Theory: Brown Sugar Gang's Relocation

Abstract

The Brown Sugar Gang relocates to mainland mushroom continent, close to Diamond City, after Wario Land: Super Mario Land 3.

Fact 1: The Brown Sugar Gang are world famous and rumor has it there's treasure to be had at their island.

Fact 3: Wario steals the crud out of the Pirates, as canonically Wario steals 90,008 - 99,999 coins plus a fair amount of the treasures from the Brown Sugar Pirates. (We know he gets his castle.)

Fact 4: Castle Syrup is rebuilt by Wario Land II and destroyed at the end of Wario Land II.

Fact 5: The 100% ending of Wario Land II shows Wario entering the Brown Sugar Gang’s treasure cache and stealing it all.

Fact 6: After Wario Land II the next time we see Captain Syrup is in the intro to Wario Land: Shake It!, stealing from a museum.

Fact 7: Captain Syrup plays Wario from the beginning of Shake It!, always aiming to swoop in at the last moment and snatch his accumulated treasures.

Argument 1: Kitchen Island does not feature the mountain ranges shown in the cutscenes of Wario Land II, nor does it have a Town for it to house Wario Land II Chapter 4: "In Town". Thus, the rebuilt Castle Syrup might be in a different place than it was in Wario Land I.

Argument 2: In Chapter 2: “Get down to the Cellar!” we see that Wario’s Island is connected to Maze Woods by a sub-oceanic train tunnel. If Wario Land II takes us back to Kitchen Island, that means Wario’s Island has a sub-oceanic train tunnel to the pirate island.
However, if Chapter 4: "In Town" is interpreted as Diamond City (as per the Nintendo building) it means Wario’s Island has a sub-oceanic traintunnel to the landmass housing both Maze woods and Diamond City, his home metropolis.

Argument 3: If we are scaling the maps based on the height of Wario, there’s only one level (Chapter 3, Section 1: "Get to Maze Woods") between Wario’s Island and the landmass housing Maze Woods, a distance Wario can swim/island hop. If this landmass is indeed Kitchen Island, this places Wario’s Island ridiculously close to Kitchen Island. Given that in Wario Land I, Mario has searched for a long time for the Golden Peach Statue, he might have found it if it was so close to Wario’s home. Thus, Kitchen Island ought to be located quite far from the Mushroom Kingdom, for Mario’s search to take so long.
There being only one level between Wario’s Island and the Maze Wood/Diamond City landmass is far more likely however.

Argument 4: The next time we see Captain Syrup is when she is after the Bottomless Coin Sack and the treasures of the Shake Dimension. She plans to use Wario to get them, and then sweep in and pluck them under his nose. Perfect for getting revenge and restocking on lost treasure.

Argument 5: Chapter 4: “In Town” features a multi-story building with a smaller rectangular roof entrance with the word "NINTENDO" and two straight rods, potentially antennae-like (Section 2: Up on the roof top).
In WarioWare: Smooth Moves, Diamond City features a multi-story building with a smaller rectangular roof entrance, with the Nintendo logo on the main building and two antennas.
Given the similarities of these buildings, they can be interpreted as being one and the same.