I have several ideas about crossover or fusion stories involving Touhou Project. I admit I want to apply rule circle nine to my favourite stories or films as much as possible (If it exist there's probably a Touhou version of it).
1. Touhou Project X Familiar of Zero in which Patchouli is summoned by Louise.
Basic premise: Louise summoned Patchouli into Halkeginia. Instead of the usual disorientation when someone who is summoned by Louise appeared in the middle of students Patchouli is fully aware of her surroundings and makes no attempt to hide the fact that she is a capable magician. She casts a translation spell on herself, erected a magic protection around her the moment Louise tried to cast the familiar binding spell and foiling Louise?s attempt to bind Patchouli to her. When she knows about commoners and nobles of this world, Patchouli do not hesitate to show them what magic she is capable of. Of course this makes Louise?s friend give her mixed evaluation of her magic abilities. In one hand she succeeds in summoning a powerful mage but in the other hand Louise cannot bind her to become her familiar.
Despite Louise?s apparent failure to make Patchouli a familiar, she wants to stay to study the magic of this realm which is based on a slightly different elemental system than what she usually uses. After Patchouli knows about Louise?s reputation she decided to tutor her because she knows that a failure cannot possibly open a portal to another world. Patchouli?s motivation to teach Louise is to study what kind of magic she's capable of and if her magic is connected to the mysterious void element. Additionally, Patchouli has studied the portal before she leaves to Halkeginia and has given Koakuma a communication device so that Patchouli can stay in touch with SDM and eventually open a portal by herself to receive supplies. When Louise begins to gather some friends, Patchouli agrees to teach them her brand of magic so Louise does not have the monopoly on advanced magic and knowledge. I would like to see the possible consequences of all of Louise?s friends getting advanced magic from Patchouli earlier in the story and how this will make a difference from the canon storyline. Maybe a bigger difference would result in Patchouli introducing a different magic system to the school than what was accepted by the church.
2. Kamen Rider Den-O influenced Touhou story (Kamen Rider Gens-O ?)
In which the role of Ryotaro Nogami is replaced by an ordinary human from the human village. This human is friendly with Reimu and her youkai friends. If the human is male he may have a crush on Reimu. Like Ryotaro, this human is capable of remaining conscious during possession and can eject the possessing youkai if she wants. The role of the Owner is replaced by Yukari, Naomi is replaced by Ran, and the Imagins (both protagonist and antagonist) are replaced by youkais (not limited from Gensokyo) although I want to see what happens if the four -taros are replaced with the four devas of the mountain. Alternatively, there is a suggestion on Danbooru that Suika replaces Momotaros, Murasa replaces Urataros, Yuugi replaces Kintaros, and Reisen replaces Ryuutaros. Then I guess Aya can replace Sieg.
Youkai and supernatural beings started to appear in the outside world and causing problems. Some of Gensokyo's youkai also disappeared. Yukari asked the human villager to solve this incident by returning the youkai into Gensokyo or if it is a new youkai, apprehend them and place them in the Gensou Liner (Yukari's train). To help in solving the incident Yukari give the human a special belt and a Suica train pass. By wearing the belt then swiping the pass in front of the belt and letting one or more of the trusted youkai to posses her the human can transform into Kamen Rider and use the powers of the youkai that possessed her. The cause of this incident is unknown and like previous incidents the ultimate goal is to find who or what is responsible and make it stop; if possible using Gensokyo way. I admit that I don't know all the important details of this story. Beside the perpetrator/cause, I am not sure I can give satisfactory justification behind why this time the five usual incident solvers cannot get involved , why Yukari needs a train to pass through her gap dimension and why the protagonist youkais need to possess a human instead of battling the antagonist youkais themselves.
My own ideas about these things are that most youkais that cause incidents in the outside world do not come from Gensokyo but from some other place/planes of existence/dimensions/reality. The missing youkai from Gensokyo has been captured and studied by the unknown perpetrator and even brainwashed to cause trouble to humans in the outside world. Since youkai alone can be controlled by the mysterious enemy, combining youkai and human abilities behind the protection of Kamen Rider armor and enhanced magitech significantly increased the chance to defeat these new enemies. In addition, the new youkais are not bound by spellcard rules so extra power and defense is a must. As to why the five incident solvers are not involved, I think that most if not all of them will repulse the idea of being possessed by youkai. Secondly, it may be better if they patrol Gensokyo, guarding against the enemy's attempt to recruit more Gensokyo youkai. Reimu is going to be needed to monitor Hakurei Barrier and Yukari is going to monitor any border breach from inside her gap dimension, using the train as a magitech observation platform and as containment for captured youkai to interrogate them. When Reimu knows about this arrangement, she demands to be on the Gensou Liner to be able to check up on her friends (the human as well as the youkais that Yukari recruits) so I think Reimu is replacing Hana's role. With this setup, it is possible for the Gensou Liner to stop not only in the outside world but also other alien places/times/planes of existence etc, that are dangerous to both human and youkai.
I don't know who can replace Yuuto, Deneb and the Zero Liner, Airi Nogami and the two frequent visitors to Milk Dipper cafe. Finally, I just want to mention that I do not like the usual motive used for many of the Touhou crossovers I have read: Yukari is bored and mess with boundaries between series to make things more entertaining for herself. So I hope if anyone is interested in turning this idea into a fanfic, they don't make it so it turns out that Yukari is the one behind it after all.
The first episode of Kamen Rider Den-O can be watched here (
http://www.youtube.com/watch?v=okH8n7EClrc) I hope this will give some ideas of what Kamen Rider Den-O is about to those that have never watched Den-O before.