Story universe idea. Earth has been isolated from a galactic federation with a prime directive not to interfere with the developments on earth since the fall of Atlantis. In the year 2030, the leaders of earth agree that they would join the federation, and its existence is made known to the people of earth. The people of the federation are all of earth humanoid types who originally seeded the planet earth. Among this federation, there was a colony established on another planet 100,000 years ago by a group of people devoted to exploring the pleasures of obesity and weight gain. This planet is secretly run by a sexually perverted scientific elite. The people of the planet believe that they are living in an absolute monarchy. The king is a figurehead of the scientific elite, but he is typically the most respected/perverted scientist among them. The people of this planet who are not in the scientific elite are bred to be fat and stupid with piggish features and a heavy desire to debase themselves. Their culture is completely manufactured by AI and is designed to instill masochistic fantasies of being controlled, fattened, and made helpless.
The scientists have devoted their lives to inventing new perversions and new creatures to satisfy their warped fantasies. Pig human hybrids, plants that capture and forcefeed humans a fattening secretion until they are helpless. They have very advanced nanobot technology (common in the federation) that they have used for their twisted experiments. These nanobots can perform medical miracles: modifying the pancreas to produce 10x the insulin, facilitating digestion by ramping up the production of digestive enzymes, reconstructing the pelvis so that it can accommodate a massive weight-gain, rewire a hypothalamus to make a person feel like they were constantly starving, giving them a full-on pig snout, gender-bending anything, restoring youth eternally, etc.
There is a massive city in the "free trade zone" that is where all space commerce happens, and is outside of the domain of the monarchy. The elite set this up to encourage trade. There is a brothel there.
The lands of the monarchy contain an "immobile asylum". This is a place where those with family members that have become too fat to care for are handed over. Once handed over, the family members lose the right to visit or ever speak to them again. The scientists perform their most twisted experiments on these disappeared people. This asylum shares an island with the monarchy's zoo. A collection of creatures designed for sexual perversions: massively obese pig human hybrids, kangaroo hybrids with massive penises, worms as thick as pythons and three feet long that live in the colons of humans muscling their way around with great force that result in massive worm infestation if not sterilized.
Story Idea #1:
The brothel places ads in weight-gain sites on earth recruiting people for their brothel that offer complete sexual slavery but also transformation into the object of their desire. A fifty-something man with diabetes decides to take them up on their offer. He is assigned to be transformed into an exact replica of Rosalie Bradford, the transformation carried out by nanobots supplied by the brothel, who have a high demand for legendary obese humans from earth.
Story Idea #2:
An earth influencer who make fun of fat bottomed women on her social media channel is subjected to a weight-gain swarm of nanobots by "the king" transforming her perspective and eventually location to the king's private chambers.
Story Idea #3:
A woman decides out of lust to be infected with unsterilized worms causing an uncontrolled infection in her bowels.