Imagine, if you will, that some of the F/sn cast are made to dress as characters from Nanoha. Imagine that Illya gets a toy bunny, blonde hair dye, and one green contact, for example. Afterwards, she retains the ability to take on her adult form - which looks a lot like Irisviel in skintight clothes. If Shirou is made to crossdress as Nanoha, it won't be a Shirou/Illya story. As <a href='http://nanoha.wikia.com/wiki/Einhalt_Stratos' target='_blank' rel='nofollow'>Einhalt Stratos</a>, OTOH (or Illya as Einhalt, for that matter, as she has <a href='http://nanoha.wikia.com/wiki/Adult_Mode' target='_blank' rel='nofollow'>about the same power</a>), its another matter.
Other thoughts involved Sakura going as Hayate, with some of the others as the Wolkenritter. Shirou as Signum might or might not allow him to Trace Laevatein, afterwards, but its an interesting thought. Due to a slight similarity of names, Rin as Rein occurred to me. Also thought of her or Illya as Shamal. Making Illya Vita could be considered cruel.
Another thought was Shirou as Yuuno, due to the latter's ability to do fairly impressive magic without a Device.
Having Rin dress as <a href='http://nanoha.wikia.com/wiki/Shario_Finieno' target='_blank' rel='nofollow'>Shari</a> amuses me, what with the 'technophobe becomes technomage' concept. Also, Rin <a href='http://safebooru.org/index.php?page=post&s=view&id=467174' target='_blank' rel='nofollow'>looks</a> <a href='http://safebooru.org/index.php?page=post&s=view&id=310933' target='_blank' rel='nofollow'>cute</a> <a href='http://safebooru.org/index.php?page=post&s=view&id=278956' target='_blank' rel='nofollow'>in</a> <a href='http://prince-charon.deviantart.com/art/Tohsaka-Rin-Motivator-160702862' target='_blank' rel='nofollow'>glasses</a>. Anyway, having at least one <a href='http://nanoha.wikia.com/wiki/Device_Meister' target='_blank' rel='nofollow'>Device Meister</a> like Shari might be necessary for them to make Intelligent Devices of their own.