Once upon a time, there were two people who loved each other with the kind of exhilarating, terrifying love that comes only once in a century. They found each other, and were whole. But like Romeo and Juliet, their love brought only bitter death; and like Heathcliff and Cathy, the grave alone was not enough to keep them apart...
Now fast forward a few decades (maybe centuries). The players would be an ordinary man and woman in the ordinary 21st century world, who, through chance or ill-advised meddling or some combination of the two, find that these two long-ago lovers are trying to possess their bodies so they can take up where they left off.
It seems to me there would be a lot of fun possibilities with this -- two strangers wrestling with a powerful, inexplicable attraction to each other, finding themselves thinking thoughts that aren't quite their own, unable to tell the difference between their own feelings and those of the possessing spirits, and on and on. So each player would sort of be running two characters, though the spirits' personalities would only really shine through now and then -- maybe more frequently as the story goes on and they get more powerful.
I'd prefer to stay away from anything fantasy-related as far as the spirits -- that is, they won't be a couple of elven wizards, but there are plenty of other historical possibilities. Anyone inspired?
Edit: Oh, and just in case it doesn't go without saying, yes, there will be sex, and not just a lot of talking about feelings.
Edit Mark II: Taken! This one's still open, though: http://elliquiy.com/forums/index.php?topic=10481.0