I have a few ideas I planned to write, but the last months it has been impossible to sit down and write something. But I can share an idea in the forum.
A new serum that does not transform you physically, only mentally. For a short amount of time, you conciseness leaves your head and the rest of you body and become trapped inside your own anus, genitals or breasts, or whatever you wish to be reduced to. You can still feel your old body as something remote, but your main world is an anus, vagina or penis. Of course one day you can choose to be your own mouth or tongue, another day a breast or your anus. Some find the time they spend in this state of mind so relaxing and arousing that they become addicted, spending more and more time like this, and eventually decides they want it to be permanent. That means they need to find someone to fuse with, since their bodies would be helpless if it became permanent in their own bodies. So they either find a dakini spawn, a normal but kinky human willing to be host or a transformee who wants someone to join them. Some have specific wishes, like becoming a pussy or an anus, others are more adventurous and don't know what body part or which gender they are gonna end up like (such as a penis on a female, or a testicle). It could be alone, or they could be more than one.
Another idea, even if it does not include transformation into an anus, is a similar elixir that twists and warps your mind, turning you into a sexual beast mentally speaking for a few hours, depending on the dose. While your body stays the same, your libido becomes animalistic and you feel none of the restraints or worries you normally feels with you human mind intact. Most of the time you don't think in the way you are used to think, but just act on your instincts. When the effect goes away, your memory is intact and you can remember what it felt like.
For some the temptation becomes too much to resist, maybe an unexpected side effect of using the elixir too often, and they want become a semi-intelligent sexbeast in both body and mind. Like a sutonian whore, or an animal with certain human traits or just something that can live somewhere that is a combination or a zoo/farm and a free brothel, spending the rest of their days in a careless existence.
An alternative explanation for the desire to permanently become an anus or a sex beast could be an unwanted side effect of the serum. When the company who produce it realize what is going on, it is pulled from the market. But the damage is already done. Hundreds of users have been affected and will end up getting transformed before they have been given any antidote.
There could also be another reason for turning into an anus. The rumor has it that if you have someone to transform into your anus, or pussy or genitals in general, these body parts will be far more sensitive than you could imagine, and sex feel more intense than it's possible to describe. It could also simply be a fashion among the wealthy. While many girls could need the money, they wouldn't need the money as an anus. So the rich women who puts out ads, looking for someone to become their anus, makes an agreement. They will write a contract where they will agree for how long they are gonna stay as an anus. A year, or maybe five years. When the time is over, they can of course not get their old body back. But their mind, which still is intact in the anus, will mentally fuse with the mind of their host. That way they will both have all the money, have some unique memories and feeling the pleasure of mentally fusing with another person. And some agrees and thinks it sounds adventurous.
Some become a full anus and has no way to communicate. Sometimes the host goes back on their promise and does not fuse with the mentally. Or when they do fuse, the mind of those transformed has become so used to being an anus in both body and mind that they can't adapt to a normal life again, and this influence is so strong that the host (now mentally fused with their previous anus) gives in an find someone who are willing to keep them as a permanent anus.
Other may still have an eyeball over the anus, which can communicate by blinking. When the time has come, they are asked if they are ready to become a human again by fusing their mind with their owner. Surprisingly many declines and is able to "talk" back that they are just fine as it is, and has no desire to become fully human again.
Or instead of rich women, two prostitutes finds out that some clients are willing to pay more for someone with a living anus. And so by flipping a coin, one agrees to become an anus for a couple of years. No living costs in the meantime, and more clients. And she has transferred all her money to her friend, which she will mentally fuse with in the future.
(It could also be a combination. An anus with a single eye, and a pair of eyes and eyebrows above the pussy, after two different girls have fused with a third one.)