* ''{{VideoGame/Touhou}}'' has innumerable doujins/fanfics detailing the BackStory of the characters, since not many characters have one to begin with, and even if they do it's still pretty vague. By far the two most common recipients are Rumia (so popular that any fan referring to [[SuperpoweredEvilSide "EX-Rumia"]] is immediately understood) and Sakuya (serial killer, former monster hunter, or Lunarian being the three main theories).
** ''FanFic/TouhouTonari'' and ''Though the Wind Cries'' are good examples of this.
* Explaining what happened to [[ChuckCunninghamSyndrome the PC-98 characters]], especially [[EnsembleDarkhorse Mima]], is similarly widespread.
* Detailing the ''future'' of characters is also common, as amongst the [[ReallySevenHundredYearsOld incredibly long-lived youkai]] there are a few humans that will [[WeAreAsMayflies die relatively soon]]. Reimu, Marisa and especially Sakuya (again) are the targets, though Sanae became increasingly popular after her arrival.
* The "Outsider Fic" subgenre, where somebody (often an AuthorAvatar, but may also be any kind of original character) somehow ends up within Gensokyo, either by accidentally/purposefully slipping through the Hakurei border (which, according to canon, is common), or by Yukari (in a sub-subgenre increasingly known as "Gappy Fics"). As a result, FishOutOfWater antics ensue as they get acquainted with this FantasyKitchenSink, and the outsider must either learn to live in this new land or find a way back home. Occasionally, they'll arrive just as an incident begins and they're forced to take part in it for one reason or another, teaming up with the canon characters. A fine example of this is [=ChibiChen=]'s ''[[http://www.fanfiction.net/s/8475931/1/Mind-the-gap Mind The Gap]]''.
** There is also the SoBadItsGood subgenre of "[[SelfInsertFic Gappy]] [[MartyStu Stu]] Fics," wherein an AuthorAvatar is gapped into Gensokyo by Yukari, makes friends with several of the residents, becomes super strong after training with Reimu and Marisa, and beats up a bad guy in one of the worst incidents to ever hit Gensokyo. These fics [[KudzuPlot start hundreds of subplots]] due to LoadsAndLoadsOfCharacters, but never answer them [[TheChrisCarterEffect before the readers just give up]]. This subgenre was [[DeconstructionFic taken apart]] by ''Fanfic/DiamondInTheRough'', showing the shallowness of these self-insert characters while addressing how dangerous Gensokyo can really be.
* The (mis)adventures of Team 9, an entirely fan-made CastHerd consisting of [[TheDitz child-like]] "low level" youkai, seems to be required for every significant story, even if only as a background detail. Indeed, the [[NonIndicativeFirstEpisode Non Indicative First Chapter]] of ''FanFic/ImperfectMetamorphosis'', and ultimately the entire story, is based around them.
* "One or more characters get turned into children" is also an oddly common plot.
* Since the setting already [[DuelsDecideEverything relies on a system of formal duels to settle disputes]], a common ExcusePlot (particularly for fangames) is "Yukari got bored and changed the system, so now the duels take the form of mahjong/soccer/etc.".
** This in turn actually IS the basis of TouhouSoccer.
* The four great Fandom-Specific Explanations of ''Touhou'' are summed up by a well-known [[MemeticMutation meme]], namely [[http://img397.imageshack.us/img397/6487/1227762377713zd9.png this doujin panel]]. (Which has actually been altered from the original, which just had the first three lines.)
** [[AWizardDidIt Magic]]: Magic in ''Touhou'' is not [[MagicAIsMagicA overly defined]], so it can be used to justify whatever need be justified.
** [[MadScientist Eirin's shady new drug]]: Eirin's tendency to make unusual concoctions of dubious pharmaceutical benefit and bizarre effect is often used to justify strange happenings. ''Inaba of the Moon and Inaba of the Earth'', [[BrokenBase generally]] considered canon, uses this one.
** [[RealityWarper Yukari is fooling around again]]: Yukari is extremely powerful, extremely lazy, and extremely capricious; the plots just write themselves.
** [[GodsNeedPrayerBadly It's a Moriya Shrine conspiracy]]: After their introduction in ''Mountain of Faith'', every single "main" game in the series (''Subterranean Animism'', ''Unidentified Fantastic Object'', and ''Ten Desires'') was either directly or indirectly caused by the Moriya trio's (more specifically Kanako's) attempts to gather more faith from the population. Suffice to say fanon took it and ran with it.
** [[MemeticMutation Templates like that are forbidden]]: Marisa says this in response to the above list. In more comedic stories, this has occasionally been used as an actual excuse for the plot.