i thought it was a common isekai story manga

Common isekai stories typically follow a set formula. There's the sudden transfer to a different world, often accompanied by a truck-kun moment (where the character is hit by a truck and wakes up in another world). Then, there's the discovery of new powers or skills, making friends or allies, and usually a grand quest to save the new world or achieve something great.

One reason could be the marketing. Maybe it didn't get as much promotion as other popular isekai novels. Without proper marketing, it's hard for a book to reach a wide audience, even if it has good content.

Well, a common isekai novel usually has a few typical elements. For example, the protagonist is often transported from our normal world to a fantasy world, either by being reincarnated or some kind of magical accident. There's often a system in place that gives the protagonist special abilities or quests to complete. Also, there's usually a power - up journey where the main character gets stronger over time.

A big trope is the 'fish out of water' situation. The character from the real world has to adapt to the strange rules and customs of the isekai world. Also, there's the 'chosen one' trope. The protagonist is somehow destined to save the isekai world or fulfill a great prophecy. And let's not forget the 'gaining levels' or 'power progression' trope, where the character gets stronger as they complete tasks or defeat enemies.

One element could be the setting. For example, instead of the typical fantasy world, it could be set in a post - apocalyptic isekai world where the protagonist has to deal with both the new world's magic and the remnants of a destroyed civilization. Another aspect could be the protagonist's unique ability. Most isekai protagonists have common powers like magic or super strength, but if the protagonist has an ability that's not often seen, like the power to control time in a very limited and strange way, it would make the story less common. Also, the way the story deals with the concept of 'returning' can be different. In a common isekai, the protagonist may not think about returning at all, but if the whole plot revolves around the protagonist's struggle between staying in the new world and finding a way back to the old one, it would stand out.

Well, it could be the character development. In some isekai novels, characters are just one - dimensional. But if this one has complex, evolving characters, that's a big plus. Also, the world - building might be more intricate than the common isekai. Instead of the typical medieval - like setting, it could have a blend of different eras or cultures.

The 'OP (overpowered) protagonist' is a big trope in isekai. If this story doesn't have that and the protagonist struggles a lot more than expected, it's a subversion. In many isekai, the protagonist quickly adapts to the new world. But if in this story, the protagonist has a really hard time adapting, like constantly getting into cultural misunderstandings or being unable to use the magic system effectively for a long time, it's different. And the 'defeat the big bad evil' plot is common. If this story focuses more on the smaller, personal battles and character growth rather than a grand, world - saving mission, it's subverting the trope.

I'd recommend checking out online manga libraries such as Comixology or Crunchyroll Manga. They often have a good selection of isekai titles. Also, some local libraries might have physical or digital copies you can access.

For me, 'That Time I Got Reincarnated as a Slime' stands out. The world-building and plot development are really engaging. Also, the characters have a lot of depth.

Well, it's a bit hard to pinpoint exactly which was the very first one. But some early examples could include 'El-Hazard: The Magnificent World', which had elements of the isekai genre.