Reincarnation Fantasy

An emerging genre, seen largely in Japanese web novels, wherein a protagonist from the mundane world dies, usually from an accident, and is reincarnated in a fantastic world with memories intact.

In a parallel phenomenon, this has also become a popular premise for English-language Self-Insert Fics, replacing the fantastic world with the setting of a work of fiction. This variant is occasionally used for Japanese web novels as well, replacing the source material with an original Show Within a Show, though these stories are more likely to have the protagonist replace a "canon" character, whereas fanfic tends toward Original Characters.

This genre has multiple points of appeal over a more traditional portal fantasy. An author has a ready-made excuse for the protagonist not to try to find their way home, as might be expected: That life is over. The central device provides a built-in reason for the protagonist to be "special", with years of cognitive development over their age peers -- rarely is the main character younger than high school at death, and they're more often in their 20s. It also gives the protagonist a built-in place in the new world's society, without having to make connections on their own or deal with a lack of cultural knowledge or even a language barrier. (This can be essential for settings where magic skill or the like is needed to make a difference on a large scale and is generally learned at a young age.) The Self-Insert Fic version also provides a handy excuse for authors to avoid revealing their real names on the Internet despite explicitly basing their characters on themselves.

Downsides include a usually slow beginning -- an infant isn't going to be able to do much -- and the fact that some plots simply aren't possible if the protagonist's old identity is to be discarded altogether. These include the aforementioned search for home, a convenient if well-worn goal that provides a natural end-point for the story, and objects from our world being brought into a fantasy setting. It also becomes vanishingly unlikely that a character from the new world will ever go to the old, or even discover its existence.

Usually makes use of A Hero Is Born and/or A Minor Kidroduction, out of simple narrative necessity.

Anime and Manga

 * Many of the Japanese examples listed under Literature have received manga adaptations, with Mushoku Tensei's being published in Monthly Comic Flapper and a number of others on the website Alphapolis.

Fan Works

 * Dreaming of Sunshine is the Trope Codifier for the fanfic version.
 * The setting of Naruto also demonstrates a number of the advantages listed above particularly well. To wield any sort of power, a character pretty much has to have been trained as a ninja from a young age, and to be wholly trusted by the main characters, it's best to have been trained by their own village. As such, the reincarnation fantasy is almost a necessity for a Naruto Self-Insert Fic.
 * In Bad Faith takes another cue from Dreaming of Sunshine: The main character becomes a canon character's Original Character twin sister, in this case Draco Malfoy's.
 * The Clan of Samsara takes the premise much further than most, with an entire ninja clan taken from across the history of the real world.
 * Tsume Yuki, of Time to Put Your Galleons Where Your Mouth Is fame, has written several stories in this genre.
 * Weight of the World gives the protagonist the very large shoes of Naruto Uzumaki herself to fill.
 * All or Nothing opts to insert its protagonist into the 1920s of the Harry Potter world, growing up during the rise of Grindelwald.
 * A Fresh Bouquet gives Lily and Petunia Evans a new sister.

Literature

 * Mushoku Tensei is probably the Trope Codifier for the Japanese version, and possibly the most popular example.
 * Re:Monster, which seems to predate most other examples (apparently having debuted online 18 months before Mushoku Tensei), differs from the norm in multiple ways -- most obviously by having the protagonist become a member of the fast-maturing goblin race instead of a human, but his original world is also implied to be a sci-fi setting, rather than a world like ours.
 * Ee? Heibon desu yo?? resembles early Mushoku Tensei, but with a softer, more shoujo feeling and more technological exchange.
 * Kenkyo, kenjitsu o motto ni ikite orimasu! features the protagonist inhabiting the role of the wealthy villain from a popular shoujo manga. Her goals largely consist of staying the hell away from the drama surrounding the heroine in high school and preparing for the karmic punishment that came her way at the manga's end.
 * An Otome Game’s Burikko Villainess Turned into a Magic Otaku is much the same as Kenkyo, kenjitsu, with a more fantastical setting.
 * Gifting the Wonderful World with Blessings! goes halfway -- the protagonist transfers worlds after death, but he gets to keep his adult body.