It seems to me that all possible constitutions have weaknesses. You could give a range of possible current best-practice ideas, each with strengths and weaknesses, and the country will pick one. Almost inevitably, they will pick the one whose strengths best address what was wrong with their previous situation. (In military terms, they're fighting the last war, not the next one.) Maybe that's the one that they most need, in the short term. It may not be the one that they need in the long term, though.
And trying to make a constitution perfect can result in some monstrously long documents. One of the virtues of the US Constitution is that it's short enough for a grade-school kid to plow through and actually understand.
