Here is a rahter long topic about L5R and how it would be possible to integrate additional historical Japanese aspects into the LCG, RPG, and Story. Several of the things included are; Yamabushi, Onmyouji, and Shinto Shrines (called Jinja in Japanese). I will provide further details in regards to each of those, as well as links to various sites that you may find useful. In addition I will discuss the Mantis Clan, and aspects about them and the historical connotations from Japan that may fit well for them. I have been studying Japanese culture and religion for a couple years now, and I am blessed to have had to opportunity to visit Japan with regular visits thanks to my in-laws being Japanese.
Yamabushi follow a religion called Shugen-do which is a syncretic faith of Shinto, Buddhism, and Toaism (this is similar in many ways to the design of the L5R combined faith of Shintao). It is a mountain faith that focuses on using asceticism to gain magical powers against evil spirits. This would be a fitting way for Shugenja in L5R to fit neatly within the Dragon Clan, mixing the Ascetic ways of the Togashi Monks with the Shugenja techniques and traditions often utilized by the other clans. In addition with the focus being on the defeat of evil spirits they would be useful against potential shadowland corruptive forces ,or for calming down angry Kami that inhabit the mountains and/or storms. In fact the religion of Shugen-do is likely the inspiration of Shugenja in L5R to an extent, as most Shugenja utilize some of the concepts to a limited degree. Albeit the Shugenja more likely fill the role of Kannushi in Shinto, more on that later.
Links: https://www.japantimes.co.jp/life/2017/10/13/travel/yamabushi-japans-ancient-tradition-mountain-ascetics-opens-public/#.WfuXLGhSzb0 https://www.britannica.com/topic/Shugen-do#ref101681 https://en.wikipedia.org/wiki/Shugend%C5%8D http://www.onmarkproductions.com/html/shugendou.html http://eos.kokugakuin.ac.jp/modules/xwords/entry.php?entryID=830
Onmyouji would be a huge boon to the Imperial Families as their own unique take on the role of the Shugenja and how to best serve the Emperor. The Five Elements that L5R utilizes are the same that the Onmyouji use in their magic (again likely a major inspiration for aspects of the Shugenja). Fortune-telling and the reading of auspicious signs for the Emperor would be a key aspect of them, in essence they would know potential aspects of the future (in game turns, perhaps adjusting dials after the fact). They would also be able to fight and put to rest angry dead, something that the Maho-Tsukai (Blood Magic users in L5R) would have reason to fear. Finally Onmyouji were known for the creation of Shikigami, basically creating a body for a Kami to serve you in various dangerous matters, be it spying, guarding, etc. This could be done in game through the creation of tokens for fate cost or something similar. Having the Onmyouji be directly connected to the Imperial Families would also add some much needed depth to the usually antagonistic Imperial Families in a more positive (potentially) light.
Links: https://en.wikipedia.org/wiki/Onmy%C5%8Dd%C5%8D http://eos.kokugakuin.ac.jp/modules/xwords/entry.php?entryID=828 https://en.wikipedia.org/wiki/Shikigami
Shinto Shrines and thus Shrines in L5R are integral to the community for a variety of reasons. Primarily in that they are one of the few things that brings together peasants and Samurai. Matsuri and various festivals are typically held by Shinto Shrines to instill community values and ideals to all people involved. They celebrate many things, be it the departed, a good harvest, graduations (Gempekku for L5R), reaching adulthood, and Holidays related to various Kami. Shinto Shrines also act as a sort of nature preserve within communities, with the nature being seen as sacred nature. In addition communities often built up around shrines, and the shrines often have a good amount of natural space around them, really creating the separation of mundane from sacred. The Torii gate is integral to the Shrine, and is not just an aesthetic choice, a Torii gate represents the entrance into sacred space, or entrance to the approach to the shrine. When you see pictures of Torii Gates within cities what you are seeing in the loss of Shrine land to the city, with the approach still remaining, not a random Torii being placed without reason. In the time period of L5R, when cities are much smaller, you would only see Torii where Shrine lands start.
Links: https://en.wikipedia.org/wiki/Shinto_shrine http://www.jinjahoncho.or.jp/en/faq/ http://eos.kokugakuin.ac.jp/modules/xwords/entry.php?entryID=248 http://eos.kokugakuin.ac.jp/modules/xwords/
Kannushi are the priests of Kami, and the keepers of Shrines. Within L5R that role is filled by the Shugenja, and very rarely do we see them doing this duty, often leaving Shrines abandoned or alone. A shrine without a priest, especially one with a sacred object to the Kami would likely lead to an angry Kami. With L5R I would love to see the more priestly aspects of the Shugenja explored, overseeing the rituals for Matsuri, doing rituals for good crop yields and bountiful harvests, really having them integrated into the spiitual core of the setting insteado being wizards and sorcerers. Shugenja should feel like they are serving the Kami (and specific Kami), when they go to war they are doing the will of the Kami, but it should be rare for them to go to war. More often they should be giving blessing to the warriors, and speaking on behalf of the Kami in court. This could make them a dangerous political powerhouse (as the Phoenix are looking to be).
Links: https://en.wikipedia.org/wiki/Kannushi http://www.onmarkproductions.com/html/shinto-priesthood.html http://eos.kokugakuin.ac.jp/modules/xwords/entry.php?entryID=1285
Ok now onto the Mantis Clan and my thoughts on them, their culture, and what makes them interesting. A lot of what I will be discussing is generalized from the old L5R with some personal thoughts. The Mantis clan in the old L5R originates from the heir of the Crab clan (Child of the Champion, and a Matsu) who was passed over for his sibling who was a bastard born of peasant stock. His mother left with him and their followers to the island of Silk and Spice. The founder of the clan utilized a Kusari-Gama as their primary weapon instead of a Katana. Looking at this we can quickly see how the Mantis clan would care little for the Katana, first due to their founder, second due to their terrain, and third due to their life upon the sea. The Mantis Clan in the old L5R near the end had an interesting theme of dual wielding peasant weapons, and a long history of using the Kama. The Kama is a cheap weapon, typically used by peasants for farming, so losing it is no big deal. It is also useful on a ship in that if can be used similar to a pick to stab into the wood to keep from going overboard. Finally it is useful in that it makes the wielder appear to look like a Praying Mantis. It is a beautiful weapon for theme, and story. The Fighting style for it would also be based around Eskrima, the Philippine Martial Art (which works for the Islands of Silk and Spice). This would give them a unique way of fighting that separates them from the other families.
Links: https://en.wikipedia.org/wiki/Kama_(weapon) https://en.wikipedia.org/wiki/Arnis http://blackbeltmag.com/daily/traditional-martial-arts-training/escrima/10-things-you-probably-didnt-know-about-the-filipino-martial-arts/
Gunpowder and Guns are probably a touchy topic for many L5R players, but I think it is an important thing to broach. Historically the Japanese were using Firearms since the 16th Century (which is around the time Samurai were a major thing). Firearms were introduced by the Portuguese and were quickly replicated by the Japanese. Samurai would lead armies of Ashigaru who would wield the Flintlock rifles of the days. An example of a major Samurai Daimyo who used it to great affect is Oda Nobunaga who nearly unified Japan before one of his own men assassinated him in a Temple (there are reports Nobunaga burnt the temple down and committed Sepuku so his traitorous retainer wouldn't have the pleasure of killing him). Gunpowder could still be fairly controlled by the samurai, without the use of cannons and expensive, but it could easily have its place in the new L5R, really setting the world apart from the previous incarnation.
Links: https://en.wikipedia.org/wiki/Firearms_of_Japan https://en.wikipedia.org/wiki/Oda_Nobunaga https://www.samurai-archives.com/nobunaga.html
I think that is a majority of what I wanted to discuss at the moment, and I think you for taking the time to read it. I would love your feedback, questions, and thoughts regarding these questions. I look forward to seeing where L5R goes from here, and I hope that I have been in some form useful.