Let's get straight to the point: right now we have 1) A new Shichibukai, 2) A handful of disembodied body parts, and 3) A weird group of animal-people headed by a blob of gas. And centaurs. If we connect the dots, we can infer that the legs, torso, and head are the same samurai, and that it was taken apart by Law. We also know that the gas-boss has something against Vegapunk, who was developing chemical weapons. Finally, the gas does NOT know that Law is on the island, as taken from this quote: "No one should even be here..." (at least that's what I interpreted). My theories are as follows. Keep in mind they are rough drafts.
1. Vegapunk's chemical weapon experiments either turned a human into the gas or made a sentient gas.
2. Vegapunk's experiments created all the crazy monsters on the island. Easy enough.
3. Law is looking to use the chemical weapons on the island to become Pirate King. He is kinda hanging out, watching the gas, and waiting for the moment to strike and steal it. He's a patient guy, remember. And that gas is working on something.
4. Law created the centaurs as minions. Suppose there were a lot of people and animals left over from Vegapunk's tests. Law arrives, and decides to create something out of them. And remember the samurai said "more allies of that shichibukai," indicating there were more.
5. Law is acting under the direct orders of the World Government to find out what this gas is going and either destroy or obtain it.
Few things though. What was the dragon doing there? Coincidence? Test subject? And how could the legs talk? And how did the legs have a different speaking pattern than the head ("buh...")?
I swear I had more, but I lost it.....think that's it....