I want to write a crossover fanfic for High School DxD, Code Geass, Unbreakable Machine Doll, Muv-Luv, Elfen Lied and Hyakka Ryouran set in an alternate history scenario that maintains DxD's canonical premise of supernatural creatures, fallen angels, mythologies and magicians all being real and hiding among humans but has some major changes: The American Revolution of 1776 succeeds like in real life but the Holy Brittanian Empire is still formed after a successful Napoleonic Invasion of the British Isles and [Brittania] is still based in Canada and Jamaica like in canon Code Geass. After the end of Napoleon, the German Confederation invades the British Isles and puts a German King on the English throne and also liberates Wales, Scotland and Ireland under indigenous monarchies. 

The Brittanians seize the US Confederate States during the US Civil War which draws the USA closer to Germany and Austria-Hungary. 

The 1868 Meiji Restoration in the Tokugawa shogunate does not abolish the Tokugawa shogunate, the samurai and the daimyo like in real life but instead makes the Tokugawa shogunate the combined local and military government of the Japanese Empire formed by the Meiji Restoration. The Tokugawa clan is split into five regent houses like in canon Muv-Luv. Industrial, economic and military development in Japan continue like in real life but the presence of Machinart from Unbreakable Machine Doll and Master Samurai from Hyakka Ryouran further boost Japans development.

The German Empire is still formed in 1871 by the Kingdom of Prussia. However Anglo-Americans loyal to the British monarchy leave to Brittania to be replaced with German and European immigrants. This weakens the Democrat Party as a totalitarian force in American politics. 

The 1911-1912 Xinhai Revolution succeeds in its goal of restoring the Ming dynasty and the Ming's reigning imperial family, the Han Chinese Imperial House of Zhu, to power in the eighteen provinces of the Han dynasty after the fall of the Qing Empire, through the Beiyang Government of the Republic of China as the Empire of China, albiet ruled by the Zhu descendants and formed in 1912 immediately after the Xinhai Revolution.

Japan helps the Qing imperial court in Manchuria seize control of the Manchuria-based Fengtian Clique lead by Zhang Zhuolin after the Xinhai Revolution, with the last Qing Emperor, Pu-Yi, taking Zhuolin's roles as warlord of Manchuria and leader of the Fengtian Clique after the putsch. 

The restored Ming dynasty in Han China is a Japanese protectorate in all but name but slowly transitions towards federalist democracy.

Guangdong and Guangxi still come under the rule of the KMT-ruled Nationalist China sometime in 2014 like in real life but Nationalist China is a independent Cantonian Republic in all but name due to the earlier deaths of both Sun Yat-Sen and Kai-Shek.

Woodrow Wilson is assassinated by Japanese intelligence before he can rise to a position of power in the Brittanian-controlled Confederate States, which along with the USA being closer to its German roots means the USA joins the war on the side of the Central Powers.

This allows the Central Powers to win the war and crush French fascism, Serbian and Russian Pan-Slavism and Brittanian imperialism. The fascist French Third Republic flees to North Africa where it remains to this day. 

The events of canon Code Geass from the 1950s-1960s AD still happen but Japan manages to repel the Brittanian invasion of its territories.

The then-Brittanian Crown Prince Lelouch along with his younger full sister Nunnally are still sent to Japan as hostages and are rescued by the Ashfords but the Japanese government, military and intelligence are made aware of Geass and the Geass Order and work against the then-Brittanian Emperor's Charles' schemes to unite humanity into one being to overthrow the gods.

Lelouch never becomes the masked revolutionary Zero who forms the armed revolutionary group dedicated to worldwide change, the Black Knights but is against Charles and his schemes. 

Someone else becomes Zero but Lelouch, who has formed a harem of girls from all over the world, escapes with his harem to a secluded island just off the coast of Japan. 

Lelouch's younger half-sister Euphemia becomes the next Empress of Brittania after Charles' death and works to improve relationships with the USA, Western Europe and Japanese dominated Asia but doesn't reveal that Lelouch and Nunnally are alive.

As Euphemia is also a part of Lelouch's harem she gives birth to his children as well, with their eldest child eventually succeeding her as leader of Brittania. 

The Vatlantis Empire based in the alternate world of Atlantis from Masou Gakuen HxH still invades Earth like in canon Masou Gakuen HxH and Earth eventually creates its own Heart x Hybrid Gears to counter this like in canon Masou Gakuen HxH.

The Institute is still formed by the Japanese government to study the diclonii like in canon Elfen Lied. 

High School DxD's protagonist, Hyoudou Issei, is the current Crown Prince of Japan by the time the story's events start in the 21th century. His status as the Crown Prince is hidden from his classmates and schoolmates at Kuoh Academy who believe him to be a simple pervert. The story focuses on Issei trying to hide who he is and getting dangerously close to failing as KKK extremists from the Brittanian-controlled Confederate States, Brittanian imperialists who hate Japan for liberating Asia and Soviet aggressors who worship both Stalin and Trotsky all aim after Issei to have his head and things get worse when he is reincarnated by his sexy English teacher who turns out to be a devil princess, Rias Gremory from High School DxD as her pawn and discovers he has the Sacred Gear with the soul of the Welsh Dragon Ddraig inside it, Boosted Gear from DxD inside him. 

But I'm lacking in ideas? How do I make the fic interesting and focus on Issei trying to keep his secret safe?