Reincarnation

Reincarnation is a common trope in fandoms where the characters have a shared destiny or are perceived that way. 
Reincarnation can be used to give characters the happy ending they were denied in canon and it gives fans more room to play with characters from a canon with a very tight timeline. Sometimes it is also used to tie a typical Alternate Universe scenario more directly to a historical canon setting (having the modern or post-canon period characters be reincarnations of the canon characters rather than just AU versions of them). 
There are also historical AUs (or modern AUs, in a futuristic fandom) where the characters are portrayed as earlier reincarnations of the canon characters; these usually end by joining up with the beginning of the canon.
Reincarnation may also be part of a mystical destiny that takes on romantic overtones between particular characters. It may also be used by writers as a way of solving the problem of only one of the partners being immortal or very long lived.
==Canon Examples==
Honoo no Mirage
In Mirage of Blaze/Honoo no Mirage the main characters are 'possessors' - essentially reincarnations of themselves who remember all their past lives (except for when they have traumatic amnesia).
J.R.R. Tolkien
Reincarnation, known as re-embodiment, is canonical for elves; for example Glorfindel in Lord of the Rings is (at least according to some canon) a reincarnation of the character from The Silmarillion. According to dwarf lore, Durin is also reincarnated.
Sailor Moon
In Sailor Moon, all main characters are reincarnated from a past life. In the Silver Millennium, Sailor Moon used to be Princess Serenity in the Moon Kingdom, Tuxedo Mask used to be Prince Endymion in the Golden Kingdom on Earth, and the Sailor Senshi and Shitennou were guardians of the Princess and the Prince.
Saiyuki
In Gensomaden Saiyuki reincarnation is canon; Saiyuki Gaiden deals with earlier incarnations of several of the main characters.
Xena
:See Uber.
In the Xenaverse it is canon that Xena and Gabrielle are destined to meet up in different lives in the future.
==Fanwork Examples==
The 100
Clexa fans in The 100 fandom created the fanon character of Elyza Lex to be a reincarnation of Clarke Griffin.
Honoo no Mirage
*Biography of the Cheated Girl by sesame seed deals with events from a prior reincarnation.
J.R.R. Tolkien
*Back to the Beginning by Adina. Legolas/Gimli played as an immortal partnered with a reincarnated character
Lois & Clark
Reincarnation is a moderately common trope in Lois & Clark stories, with Lois Lane being reincarnated as the soulmate of an effectively immortal Clark Kent.
*The "Home" Series, by Nan Smith
Merlin
Reincarnation is a popular trope in Merlin fandom.
*Accidental Memory In The Case Of Death by derryere - Merlin/Arthur reincarnation fic and one of the most commented-on stories that came out of the Box of Magic Big Bang in 2009. In 2010 nerjaveika created a stunning vid adaption, Accidental Memory In The Case Of Death (Merlin vid).
*There Are Legends by Ras Elased - Merlin/Arthur reincarnation vid. Summary: "The legends say that Arthur died, and he waits in Avalon to be reborn. The legends say that Merlin is immortal, and he lives on, waiting for his king. But what if the legends are wrong?" 
*These Aren't My Toys (I Just Play With Them) by misswinterhill - Merlin/Arthur AU where Merlin is a Camelot Collectable action figure and falls in love with the other half of his limited edition run. The story says interesting things about canon, fanfic and Arthurian Legend and it uses the reincarnation trope in a fascinating way.
*Steam by new_kate - Renaissance-era post-apocalyptic steampunk AU reincarnation fic set 300 years after canon timeline. The story was written for Paperlegends, the Merlin Big Bang challenge in 2010. 
*Age, and Other Artificial Concepts by Trojie. Crossover with Narnia. Arthur/Merlin, Caspian/Peter
*Genetics series by Lisgreomg. Crossover with Sherlock. Merlin Holmes, the youngest Holmes brother, reaches out to Sherlock for help finding a friend that has gone missing. 
*The Goose Girl by [rosa_acicularis]. Nimueh returns to Camelot. AU after series one.
*Postremus Regum Britanniae by definewisdom. 
*Reverse, Rewind by xaritomene. 
*True North by kroka-refs. Arthur/Merlin
*We Are Eternal by kanerame. 
*You Shall Know The Truth, And The Truth Shall Make You Free by vampiric_mod. The trouble with reincarnation – and actually remembering your past lives – was that it got a bit repetitive after a while. Arthur/Gwen, Arthur/Merlin
*A list of reincarnation links and reincarnation themed story searches on the merlin_finders community.
Saiyuki
There are fanworks set during Gaiden, as well as stories set between Gaiden and Gensomaden and during future reincarnations after the end of the series. These are often set in the modern day. Sometimes the characters are reincarnated as characters from another fandom. Sometimes the characters are born into female bodies.
* Wake Up by Viridian5. Future incarnations, meeting up again.
* Bargaining at the Gate by Dracostella. Gen. Immediately post-Gaiden, between one incarnation and the next. 
* Reunion by Veszelyite. Immediately post-Gensomaden, between one incarnation and the next.
* Heaven and Hereafter by harukumi. Immediately post-Gensomaden, one of the characters is not permitted into heaven.
==Links==
*Works tagged with Reincarnation on AO3
Category:Glossary