I'm thinking of doing an Isekai story. I thought up this idea after I found out about Cheat Slayer and it's shallow parody nature. A guy gets hit by a truck and reincarnates as a half elf/half demon. His new father would be an Isekai Slayer, who's job is to hurt and kill people who have crossed over from our world after numerous people from Earth have screwed everything up, ranging from well intended heroes who turned out to be bad at ruling or leaving descendants who become tyrants to becoming a Demon King. The protagonist meets up with other people who got Isekaied and forms a team. Their leader would be a Kirito expy who reincarnated as his game avatar. He would also have reincarnated in five previous worlds. He could also have lost a harem to the Demon King. The rest of the group would be based on expies of the protagonists from Wise Man's Grandchild, My Next Life as a Villainess, Re:Zero, That Time I Got Reincarnated as a Slime, I've Been Killing Slimes for 300 Years and Maxed Out My Level, Rising of Shield Hero, No Game No Life and So I'm a Spider, So What? Note that not all of them would be reincarnates. Some will have crossed over by suddenly finding themselves there while walking home from a convenience store at night or reading a book. The Catarina Claes expy would have reincarnated as the daughter of a corrupt duke who fears the consequences of a revolution. The Subaru Natsuki expy would be a cynic (because his story is a deconstruction) and would be in a vegetive state due to the trauma of so many alternate timelines. I might make him a cross between Subaru and Naofumi from Shield Hero. I'll probably make the Shin Wolford expy a cross with Harry Potter since I was gonna do a Harry Potter story as a rebuttal of Alan Moore's take before finding out about a fanfic called League of Extraordinary Gentlemen Tempest Rewrite. Might give him elementals of Edward Elric even if Fullmetal Alchemist is not Isekai. The Demon King could be an expy of the protagonist of Overlord, who also reincarnated as his game avatar, with an expy of Tanya the Evil serving him. Another antagonist could be a fire based version of Aqua from Kono Suba who would pretend to be a reincarnate (she actually killed an expy of Kazuma and took his place) to infiltrate the Isekai Heroes. Because Konosuba Suba is a parody, she'd take the situation less seriously than the others. Might make her a cross between Aqua and protagonists of other parody works. Might make her the final boss as well if she's a god.
I wrote something in this vein just recently. Except it was more focused on the aspect that most isekais have of the world bending over backwards to accommodate the protagonist. In my case I just had the protagonist actively do that himself with the power of Rock and Roll. I also wrote it as a parody: https://www.deviantart.com/artificer-urza/art/Isekai-Rock-931904094
The heroes from Master of Ragnarok and In Another World with My Smart would work as villains since many of the criticisms of the isekai genres actually apply to them. Women fall in love with Yuuto upon meeting him for flimsy reasons and his harem has little girls. I'm not sure how I'd go into that bit. Are underage characters fine if the lewd stuff done with them if done offscreen, right? He'd be going around adding women to his harem, while killing any male who's gets in his way. Tonya could be a Mary Sue parody.
Reversed isekai ideas You playing hentai game during a storm and sudendly lightning hit your house, this make a blackout and after you find flashlight a switch it on, all girls from the game are in your room, naked and the youngest pulling your pants down.
Funnily enough the next story I plan to do is about a guy getting trapped INSIDE a game and having sex with the girls inside.
You died when you save your childhood crush from being killed by train, reincarnated as wererabbit that is instantly killed by hunter and reincarnated as your childhood crush's favorite panties.
It depends on how it is depicted. Though it would already violate the rules if an underage character is present while "lewd stuff" is done to another character. When creating Fanfiction, you can simply modify the lore. There is no need to stick to the canon material.
It's an original work that the that uses expies of established isekai characters. The Yuuto expy is meant to a deconstruction of the character, so changing it too much might miss the point. Is it okay if some of his harem being underage is just mentioned?
