Dungeon Mastery: Building an Isekai

If you are going with an Isekai for your Fiction or D&D game it needs five things:

1. It needs to be happening in the Other World and the Earth World at the Same time.

So when the school bus experiences a warp transporting the students to Summer camp and the zero level PCs are dumped into the Isekai of their Nightmares, it literally cuts the bus in half leaving the players with characters sitting in the back of the bus and in a position to investigate the techno-organic gate that straddles the road, and their cell phones can communicate through the gate and are in a position to seek help, but also the second group of characters in the front of the bus who are now in the red wastes of Aard where there is no water or food that they can find.


2. It needs to be very different to the world they left behind. Its hard to achieve 'Alien' to the Characters from modern earth where knowledge of obscure landscapes, though unexperienced by a band of school kids from suburban Tokyo might be well known to them thanks to the internet and education.

Unlike Earth, Aard is a world where the PCs can survive by working together or through betrayal. The Red Wastes are brutal, where the soils are rich in iron that magnetic fields are going to create electrical fields. Electrical Storms are common. Dust Storms involve walls of dust a mile high rolling across the landscape.


3. A magic system exists that can develop over time.

The Red Wastes magnetic fields generating electric fields that can be made use of by intelligent characters. 
A Steel Rod can be thrown as a spear using a Womerra.
Because of the high electrical storm nature of the Isekai region, Lightning will go to ground through a spear of steel that has penetrated the target from above causing lightning to ground through the target.
Given the red soil is used to soak up sweat and mixed as mud in hair and coat the body to protect it from sunburn and reduce moisture loss.


4. Multiple Characters interacting and conversing in the isekai setting.

The Bus Driver/Teacher, and twelve Students are travelling up into the mountains to a camping outing.... the Teacher, and five students are pulled into the other world. Seven high school students are left injured on a mountain road. 

Characters in the Other World

• Miss Kobayashi (Teacher, involved with same teacher as Ai)
• Mio (Student, Photography Club President)
• Rin (Student, socially isolated, Manga Artist)
• Yue (Student, Athletic, Class President, bully)
• Umi (Student, socially isolated and quiet, regularly bullied by Yue)
• Ai (Student, secretly pregnant with a teacher's child, was planning to kill herself on the trip)

Characters in the Real World

• Miko (student, Involved with Jin)
• Jin (student, only male in class, involved with Miko)
• Reiko (student, Jin's previous interest)
• Tae (student, socially isolated Goth girl)
• Kanna (student, music club member)
• Akkane (student, music club member)
• Suki (student, music club member)

5. Self Awareness that the characters are Survivors as opposed to Heroes. Some characters establish them selves as 'survivors' early on. Others prove increasingly selfish over time.

Jin, who moments earlier had been fondling his fellow student Miko's thigh watched the front of the bus vanish. They are propelled forward from the exposed rear of the bisected bus. Thrown through the air toward the certain injury that would soon be shared by both, Jin grabs Miko and adjusted his position putting her between himself and the rock approaching outcrop using her to cushion their deceleration. Her impact on the rock comes with a terrible crack that caused her thighs to go limp in his hands. She will never walk again.

Yue, faced with starvation or living in a world where there is no rescue, murders Rin in the middle of their first night and prepares Rin's remains as their food source. Her actions lure a mud worm out of the soil which proves to be an alternate food source. She keeps Rin's manga art out of guilt, reading it in secret. Carrying nothing but Rin's Manga Yue flees the group. She finds that Rin's manga contains the dirty little secrets of a number of students and teachers including Ai's pregnancy and the father of her child.