Monday marked the 225th anniversary of the Constitution of the United States, publicly unveiled in Philadelphia amid considerable fanfare. As Americans think back on the last two and a quarter centuries, we should also think ahead by the same amount. What will and what should the Constitution look like 200 years hence? Or if that seems too mind-bendingly futuristic, what will and should it look like in 2020 or 2121?

The most powerful portents of the future are to be found in America's existing state constitutions, the proverbial laboratories of American democracy. These 50 documents have converged to form a distinctly American model of governance — call it "American exceptionalism," if you like. For example, unlike the regimes in various democratic countries around the world — England, Germany, France, Israel, India, Australia — almost all 50 states follow the same basic formula, featuring ratified written constitutions, bicameral legislatures, chief executives who look remarkably like mini-presidents and robust bills of rights enforceable in ordinary courtrooms.

This is our basic American model, and it's not going anywhere. Proposals to amend the federal Constitution are taken seriously when they fit within this framework — and especially when they have already been adopted and road-tested by the states. A brief history lesson: The federal Bill of Rights, ratified in 1791, tracked various pre-existing state bills of rights; the federal Reconstruction Amendments of the 1860s, ending slavery and protecting free blacks, borrowed from the best constitutional practices of antebellum free states; and the federal Woman Suffrage Amendment prevailed at the continental level in 1920 only after women had won the vote under many state constitutions.

Indeed, the Philadelphia Constitution of 1787 to which we wish happy birthday was itself built upon state constitutional templates. In their decision to put the federal Constitution to a special ratification vote of the people, the framers copied the models of the Massachusetts Constitution of 1780 and the New Hampshire Constitution of 1784. The idea of a federal census borrowed from the Pennsylvania Constitution of 1776 and the New York Constitution of 1777. The elimination of property qualifications for federal public servants likewise borrowed from Pennsylvania, as did provisions for all federal lawmakers to be compensated from the public fisc. The broad outlines of executive power combined the best of the Massachusetts and New York constitutions. Various elements of judicial independence drew upon state antecedents, as did the U.S. Constitution's basic commitment to trial by jury.

So much for the past 225 years. Let's now think about the next two centuries. How could we spruce up our founding documents based on changes the states have made to theirs?

Many states have term limits for state legislators and also allow voters to recall corrupt or incompetent state lawmakers in certain situations. This invites us to think hard about whether the federal Constitution should provide similar checks against Congress. Or consider the fact that no state has an upper house in which each county has two seats, regardless of population. Perhaps the Senate could be changed to be more proportionate. Imagine, for instance, a federal amendment giving each state at least one senator and capping even the largest state at eight senators. This would keep the Senate at roughly its present size and would respect the existence of the states, while dramatically reducing mal-apportionment. (There is, of course, a special rule in the U.S. Constitution making it particularly difficult to modify the apportionment of the Senate via a constitutional amendment. But nothing prevents future amenders from preserving the existing apportionment of the official "Senate" while transferring much of the Senate's real power to a newly created, more proportionate, entity — call it Senate 2.0.)

Consider also the way that the states elect governors — directly by the voters, one person, one vote. If the federal Electoral College is so good, why does no state closely follow it? Let's scrap it. And if naturalized Americans like California's Arnold Schwarzenegger and Michigan's Jennifer Granholm can be trusted to serve as governors, why not amend the federal Constitution to allow them to run for president? (In fact, in 2004, the Senate held hearings on just such a proposed amendment, sponsored by Sen. Orrin Hatch, R-Utah, and strongly endorsed by several congressional Democrats.)

But is it realistic to think that current entrenched interests would ever allow sweeping amendments to prevail? For example, why would Wyoming ever agree to change the way the states are represented in the Senate, given how the current regime favors states with small populations? Or why would our representatives in Congress ever support an amendment to limit their own terms? The key answer is a long time-delay between the vote on a visionary amendment and its start date — the sunrise rule.

A close look at the original Constitution reveals a few clever uses of this device. Although the Deep South refused to give up the power to import trans-Atlantic slaves for the first 20 years, the region was willing to allow Congress to ban importations beginning in 1808, and forevermore. Had the framers been equally clever in the use of sunrise rules on other slavery-related issues — for example, had the original Constitution prohibited slavery in all western territory after 1808, or prohibited three-fifths apportionment credit for all slaves after 1808 — perhaps Americans might have ultimately ended slavery without the unspeakable carnage of the Civil War. States in the founding era, including Pennsylvania, Connecticut and New Jersey, did use sunrise rules to achieve gradual abolition. Slaves at the time were not liberated, but their children walked free.

In the same spirit, amendment-minded Americans should imagine ourselves today as representatives of 22nd-century posterity, tasked with the awesome challenge of framing just rules for that society even though we won't be here to see it. As modern American constitutionalists focus obsessively on the deeds and words of 225 years ago, shouldn't we spend at least some time thinking about what we want two centuries into the future? Much of American constitutional law remains to be written.

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Amar is a law and political science professor at Yale and the author of "America's Constitution: A Biography."