[@Sierra] I think the simplest thing to do in the case of your spirits would be to simply make it so that even though there are twelve of these spirits, they can trade their responsibilities with other spirits, or are perhaps reborn every time they take on a new "master." But since that ship has sailed, your players are going to have to make due with these other spirits. Unless it's not too late to write up some mumbo jumbo in the lore. I specifically design my RPs so that problems like this never arise. 

But even so, nothing dulls the exhilarating feel of death like a bunch of pointless inactivity kills. You might like trying this system if you have some characters like that. I'm actually surprised at how many of my [i]old[/i] players are willing to take up another character. But you can always slow the dull of the death knife by giving the physical characters reasons to abandon their quest, or otherwise drop out of the story without dropping dead. Then that just leaves you with figuring out what to do with these spirits.