John Adams pointed out the biggest failing of the Constitution:
Our Constitution was made only for a moral and religious people. It is wholly inadequate to the government of any other.
If ANYONE should have been aware of how immoral people can destroy a country, it's the founders of this country; that's why they LEFT England.
Two changes I would like to see in the Constitution: Judicial review BEFORE a bill is signed into law. The idea that Congress can pass whatever law they want, then we have to spend the effort to fight it in court afterwards is terrible. Related to that would be punishments for anyone passing unconstitutional laws. First being a censure, second being a fine, third being immediate removal from office and barring from ever holding a position of public trust. I think that would have squashed a lot of the unconstitutional laws we see from ever being passed.
Of course, the problem there is, who would be enforcing these punishments. How often do members of our government investigate themselves (or their corporate friends) and find no wrong doing? Yes, I know that was the purpose of the 2nd Amendment, but that has its own complications.