Idea that may or may not make it into the inflation D&D book that I may or may not make ;3
I'm imagining the concept of a King and Queen Slime being almost a parasitic relationship. The Queen Slime enters the body of the host, mostly seen as them having "fun," engaging in usual slime-person style intercourse. Moving through the body, inflating various body parts, before stopping in the stomach and making it as large as possible to immobilize the host before moving out of their sex organ to jack off the host. Doing so has the host cum out the Slime Queen and turning the host into a Slime King, drastically increasing their libido and desires to cum into others.
Said Slime King now only cums Sliemen (Slime Semen), ejecting a large amount of extra slimy cum. When this is done to another person, the Sliemen fills up whatever hole was came into, (For what's next, better hope their first experience wasn't a blow job) inflating the gut to look roughly pregnant sized. But not for long. As the Sliemen then grows inside its new host. Over the course of the next hour, they continue to inflate. Once at full size, the semen coating evaporates, making the host incredibly horny in order to make the next part very enjoyable. Finally, baby slimes start popping out of the hole the Sliemen entered.
If a host takes the Sliemen of a King Slime multiple times, then the max size and how many slimes are produced are increased each time. Roughly around the 5th time, the host doesn't inflate at all. They feel the Sliemen enter and fill their body as normal, but it doesn't stop at their gut. It continues onward, making their whole body feel full of slime. The feeling then starts moving outward until the host notices their skin changing texture. They have been successfully morphed into a Slime Queen and now has the desire to be within people. To fill them up. Get them as big as possible. Then be cummed out.
Slime Queens are indistinguishable from other Slime People and have been transformed fully into living slime. Queen is of course just a title and gender is rarely seen making any difference.
Slime Kings, however, are still seen as whatever they were prior. Though usually humanoid. Slimes may still go into and Inflate a King, but it feels more like a hug from family than anything else. Sometimes, Kings are noticed to carry more "water weight" after becoming a King. Mostly due to slimes not wanting to leave their body. Though mostly preferred to be male hosts, female Kings do exists. Any Cis Female Slime King finds that any orgasm now Shoots out Sliemen. Said Sliemen is still part slime and can move around a little bit and squeeze into tight spaces. Sliemen doesn't last too long, so any King with a Vagina wanting to produce more slimes will want to make sure to aim onto the desired body, preferably close to an entry hole. It has also been noted that there have been instances of a Female King cumming into a Dick. This caused the Dick Haver to swellin their balls, rather than their gut. Practically ejaculating the Slimes once ready.