> Would it be reasonable to make it so that sets are 
> always created with the definition order?

No, it would not.  We would also have to maintain order across set operations such as intersection which which would become dramatically more expensive if they had to maintain order.  For example intersecting a million element set with a ten element set always takes ten steps regardless of the order of arguments, but to maintain order of the left hand operand could take a hundred times more work.