You'll excuse this bit of historical indulgence: I've been curious about this for years, and realized I had the tools to accomplish it quickly. In 1784, just after the US had established the Mississippi River as its western border by treaty, Thomas Jefferson submitted a proposal to the Continental Congress to divide the Northwest Territory, then Indian land almost entirely unsettled by Europeans, into ten states, mostly as blocks by lines of latitude and longitude. To these he gave mostly Latinate-Indian names. With this proposal there exists a map, mostly but not entirely congruent with the proposal, but encompassing the entire trans-Appalachian region, including south of the Ohio River.
Jefferson's proposal was shelved, and in the event, the Northwest Territory became 5 1/2 states (Ohio, Indiana, Illinois, Michigan, Wisconsin, and part of Minnesota) larger and slightly less regular in shape. What I was wondering was: if the proposal had been adopted, and nothing else had changed, where would his states be on today's maps, and what would be their populations?
I found an adapted modern map of the Northwest Territory showing Jefferson's states. Note that the map has moved the northern boundary of the southmost tier north by half a degree of latitude. Without doing so, the state of Pelisipia, which in Jefferson's map lies south of the Ohio, would in its Northwest Territory incarnation be reduced to almost nothing.
It was a brief task to measure these lines on more detailed maps, normalize them to the nearest county boundaries, tag the counties on a 2010 census database, and add up the results. Because Jefferson had ten states where we only have 5 1/2, only two of his states are as populous as any of the actual states. From north to south, they are:
Sylvania: NE Minnesota, northern Wisconsin, and the UP of Michigan; urban centers Duluth and the northern suburbs of St. Paul; population 2023 th.
Michigania: central Wisconsin plus St. Paul; urban centers Milwaukee, Madison, and St. Paul; population 5013 th.
Cherronesus: northern parts of the Michigan Lower Peninsula; urban centers Flint, Saginaw, Muskegon; population 2486 th.
Assenisipia: northern Illinois and associated bits of Wisconsin, Indiana, and SW Michigan; urban centers Chicago and South Bend; by far the most populous of the ten; population 12,239 th.
Metropotamia: most of southern Michigan, NW Ohio, and a bit of NE Indiana; urban centers Detroit, Toledo, Grand Rapids, Fort Wayne; the only other large-population state; population 8580 th.
Illinoia: most of central Illinois and central Indiana; urban centers Springfield and Indianapolis; population 3786 th.
Saratoga: most of central Ohio and part of eastern Indiana; urban centers Columbus and Dayton; population 4486 th.
Polypotamia: most of southern Illinois and southern Indiana; urban centers East St. Louis and Evansville; population 2788 th.
Pelisipia: southern Ohio and nearby parts of Indiana; urban center Cincinnati; population 2406 th.
Washington: eastern Ohio; urban centers Cleveland, Akron, etc.; population 4606 th.