I definitely don't have a problem with people coming up with NPCs. If you're going to make them a CS, however, that's kind of where a certain unwritten rule I came up with comes into play-- that the NPC will start hearing the whispers in the night. 

Here's the trick of it, though-- none of your characters are really obligated to do as the voice says at this point. And when your character is created is sort of when the whispers begin for them in a sense. So if we were on day 10 in the IC and you threw in your new character, one of four things would happen:

-Your character didn't start hearing the voice until just the night/a few nights before
-The voice had come to them just like everyone else but they had ignored it and not gone to the festival.
-They had been hearing the voice for some time only it was telling them something different compared to what everyone else was originally told (to go to the festival)
-They have yet to hear the voice at all but eventually will

(We would kind of work something out via PM most likely.)

You can make and control an NPC for however long you like, but if it seems to me that you're playing him/her more than your actual character or they've become a pretty substantial part of the group, I'll probably ask that either the NPC fades out soon or you make a CS for them. This isn't so much of an invitation to go careening down a ton of player-made side-quests as it is an invitation to make side characters and small situation/occurrences that will add some believable zing to the overall story/quest.