I was thinking something along the lines of:
If you win the game, you get the right to make a deal with the devil.
If you want your wish, you need to trade a soul for that wish. But it doesn't have to be your soul, it can be that of another person. SO if you manage to keep hold of a soul of someone else, you can trade it for your wish without risking your own place in heaven.
If only have your own soul at the end of the game and make a deal, you will be allowed to live a full life, but after that your soul will be collected by the devil.

That is, unless you can come up with a rule change that would make things more fun, I've been out of the running for some time so I am open to all suggestions.