One of the many versions of Zelretch out there comes into contact with the BtVS episode <a href='http://buffy.wikia.com/wiki/Halloween' target='_blank' rel='nofollow'>Halloween</a>, by way of one of the many (many, many...) Alternate Halloween fanfics out there. Reading a few more, he decides to try an experiment: see if he can duplicate not only the Halloween spell itself, but the ability shown by Xander in canon, and lots and lots of people in fanon, to retain abilities of the costume-entity, after the spell has been cancelled. To maximise the knowledge gained, he talks a bunch of other Zelretchs into doing the same, in their Worlds. They will use mostly the same subjects, to observe how those subjects are altered differently. Specifically, the students of Homurabara Gakuen (though you could use the cast of Tsukihime, instead, if you like - not all the Zel's agree on who the most amusing victims are) are going to get a costume party on Hallowe'en, 2003, just a few months before the Fifth Holy Grail War - with special attention given to Rin, and those associated with her. Due to timelines that flow a little faster than others, the Zel's are mostly aware of how the Fifth Heaven's Feel will go without their interference, and so are curious about how it would go with this change. For example, suppose he sends Shirou a King Arthur costume, with, say, Rin as Guinevere, and Sakura as Lancelot?
If you especially want Ilya to be involved, Zel could arrange it. She summoned Herakles about a year before the Fifth War, but I'm not sure when she arrived in Fuyuki. Say, Shirou dresses as Kiritsugu, and Ilya as Iris (which I hope leads to her keeping the grown-up, not-going-to-die-in-a-year body, afterwards).
Crossovers are acceptable, but may need some adjustment to get them to work. Naruto, for example, has 'ninja' (or what they call ninja) apparently using the Second Magic at genin (beginner) level, which could have nasty effects on the readers' SoD.
If you especially want Ilya to be involved, Zel could arrange it. She summoned Herakles about a year before the Fifth War, but I'm not sure when she arrived in Fuyuki. Say, Shirou dresses as Kiritsugu, and Ilya as Iris (which I hope leads to her keeping the grown-up, not-going-to-die-in-a-year body, afterwards).
Crossovers are acceptable, but may need some adjustment to get them to work. Naruto, for example, has 'ninja' (or what they call ninja) apparently using the Second Magic at genin (beginner) level, which could have nasty effects on the readers' SoD.