Basically it's based upon Koei's video game series, popularly known as "Dynasty Warriors". It has several spin-off titles such as Samurai Warriors, Dynasty Warriors Gundam, Warriors Orochi, etc.
This one is focusing on it's main series, Dynasty Warriors. Which is a re-telling of the famous "Romance of the Three Kingdoms" Story. Based in China, three Kingdoms struggle to unify all of China. These Kingdoms are Wei, headed by Cao Cao. A powerful Noble, brilliant man of war. And dubbed "The Hero of Chaos".
Wu, headed by many. The first is Sun Jian, the Tiger of Jiang Dong. Early in the conflicts before the Three Kingdoms truly began, Sun Jian fell to an ambush. Leaving his son, Sun Ce as the heir. Sun Ce was named "Little Conqueror" for his young age but surprising knack for leadership and domination of his foes. Sun Ce apparently passes away due to illness, leaving Sun Quan as the eventual ruler of Wu.
Finally, there is Shu. The Kingdom that holds the "least" amount of power. Their ideals are more peaceful and they serve the people. Liu Bei is their leader, and he is an Oath Brother with the two famed warriors Guan Yu and Zhang Fei.
I'm sure you can find the rest of the information yourself. Back on track... Gensokyo Warriors is basically a spin-off series. Instead of starring warriors of history, it stars the characters of Touhou. These are the Factions:
Human Village, Keine: Replaces Han Empire
The Human Village is integral to the plot. A relatively simple task of neutralizing a minor Youkai Dissident Threat explodes into a full-blown incident as many Humans disappear or are injured, and Keine herself is forced to rest due to many grievous wounds. Her condition renders her unable to answer any questions as to who was behind the incident. And the Humans have scarce information on the subject.
The Hakurei Shrine, Reimu: Replaces Dong Zhou
The incident at the Village soon catches Reimu's ears. She rallies those able to fight and soon enters a battle with all of the Youkai of Gensokyo, demanding answers from them over what happened. With Keine unable to answer, Reimu deems this the only valid course of action. As such signs of power have worked in the past when solving Incidents.
Team (9), Cirno: Replaces Yuan Shao
Cirno is, surprisingly, one of the first to retaliate to Reimu's actions. She gathers up several Youkai into a makeshift army to do battle with the Hakurei Shrine. Why? She wants to be named the Strongest and greatest General of all time. Meanwhile, the Youkais join her because they wish to prove themselves innocent.
Scarlet Devil Mansion, Remilia: Replaces Wei
It goes without saying that with the many skilled members under Remilia's command, as well as her vast resources and calm, charismatic surface makes her a powerful leader. With the brutal efficiency of Sakuya and the huge strength of Meiling, they are major contenders. She is one of the two personally recruited by Cirno's Coalition against the Shrine's Actions.
Eientei, Eirin: Replaces Wu
Eientei is the other of the two Factions that is personally recruited to join the Coalition against the Shrine. Each member contains a brilliant mind for tactics. What they lack in sheer size and resources, they make up for with a surprising efficiency on the battlefield. They partly join the Coalition because many of them are Youkai, but also because... Mokou is with the Hakurei Shrine.
Moriya Shrine, Suwako: Replaces Shu
Moriya Shrine actively takes in Youkai Faith. Whether or not it was possible for a Youkai to be behind the incident is not the concern. They are opting for a peaceful resolution with the Hakurei Shrine, and are protecting the Youkai who are simply trying to fend for themselves. They volunteered to join Cirno's Coalition against the Shrine.
That's all the main factions. There's another one that comes into the game later. It's the main boss's Faction. This song will probably give away who it is.
That's the idea I had.