The Nemesis Sheet
It is said to appear between rivals who share true hatred, crossing worlds to find them both. When one receives the Nemesis Sheet, their enemy in a parallel realm receives it as well. Here are its rules:
Rule 1 - When the Sheet manifests before you, know this: Your bitterest enemy, in a parallel world, holds its twin;
Rule 2 - If you choose to use the Nemesis Sheet, your foe will become a beautiful woman. There is no other outcome;
Rule 3 - You may write the how, the when, and the what-she-remembers. You may even command reality itself to forget the man she was;
Rule 4 - Your Sheet is powerless until your enemy's is activated, and vice-versa. To use it is to know that they, too, have chosen to use it on you.
----------------
How it would work:
So this serie would work something like this:
Page 1 - We are presented Two Enemies, and in different timelines each receives the NEMESIS SHEET;
Pages 2&3 - Shows what happens on Timeline A - Character A transforms Character B;
Pages 4&5 - Shows what happens on Timeline B - Character B transforms Character A;
Amusingly, Rule 4 means that any transformation you go through is SELF INFLICTED.
Both parties are assholes, both are assholes enough that they don't mind screwing alternate versions of themselves as a "Trade Off" - making them partialy to blame for their own situation, and both get an ending as guy/girl.
----------------
Example:In the setting we have an asshole class pres that is a hardass and power trips - and a delinquent bully that likes doing delinquent bully things;
In Timeline A
Delinquent gets the Nemesis Sheet: Delinquent writes that "all class presidents must be curvy hot girls" and writes down "every time someone breaks the rules, the class pres must do lewd punishments" and etc. With the delinquent being delighted to be able to break the rules and the class rep now has to do all these embarrassing things.
In Timeline B
Class Pres gets the Nemesis Sheet: Class pres writes "Every time Delinquent tries to break school dress code, he will dress like a gyaru. Every time he tries to fight someone he will suck them off. He shows his hatred by doing lewd things". Class pres now knows that the more Delinquent tries to defy him, the more he will fall for it.
And then I can keep doing it again and again with all sorts of archetypes and characters. Sometimes I could even sprinkle something like "their new waifu makes them a nicer person".
----------------
From time to time I kind of want to do some transformations, where one character is maliciously transforming the other, but I have an odd relation with that approach. While I have no problems reading stuff like that (and even find it hot), doing it without any commonplace to the asshole transdorming the other others also doesn't quite sit well for me. It's why most often my work have forces outside of the character's control transforming them.
I also often have lots of one off ideas for transformations on my back burner, with several different designs and concepts I'd like to draw.
I think I got a way to solve both of these in one swoop.