Go - Resource penalties (at least until late game on Oil). It makes zero sense that if I fairly traded for horses and built 16 units that after the trade I receive a penalty unless I disband them. Same with units built from Iron. In reality this sort of thing would be insane. The only resource that makes sense with this penalty is Oil. Of course the A.I (or player) receives zero penalty for the gold I paid.) This would only apply with traded resources. --Losing your own has a penalty. Add - Religious intolerance that wears down over time. Different religions should have an impact on diplomacy much more than it does now until the modern era where degrades heavily and should have little impact whatsoever by the information age (as long as you have not recently been at war).