Constitutions: Principle, Not Policy
The way I see it, a Constitution should be short and sweet.
It should enumerate a few fundamental principles. The fewer the better. Beyond that, a Constitution should do... nothing.
It's for governments (ideally, democractically-elected governments) to set policy, within the bounds of the principles enumerated in their Constitution. Building policy into a Constitution is, to my mind, an surefire recipe for disaster.
But if the European Union outlasts the United States of America, I'll change my mind.