The only rule (for this exercise) is that your rule is a rule, not an idea/concept. Give us a fully-worded rule.
For example, please don't respond "if it's more than mildly interesting then don't allow it". That's not a rule, and you will only understand how difficult it is to write a new rule until you try to write one yourself. It's fine if the rule needs a longer explanation, such as with Rule 6 (it is quite long, and also has a longer explanation of it linked in the sidebar).
Let the rule-writing commence!
on another note, we are still reviewing mod applications