While users were always able to change building graphics in previous SimCity games, using tools like the SimCity 2000 Urban Renewal Kit (SCURK) or the BAT tool for SimCity 3000, the BAT tool for SimCity 4 allowed, for the first time, to actually add NEW buildings into the game rather than just swapping out what was already there, providing, in theory, for a limitless amount of buildings. Naturally, imaginations zoomed.
In 2004, a team was set up to create buildings and landmarks in Michigan, in what would be known as the Barry Sanders Project, named after one of the greatest Detroit Lions players ever (and one of the best running backs in NFL history), Barry Sanders.
Ultimately, about three dozen buildings were released under the "new" BSP from 2007 to 2010. In 2013 BSP was once again re-booted as "mipro" ("The Michigan Project"). This page once had pictures of an in-progress Hudson's department store (from November 2010) but because they were hosted on University of Michigan's servers as a student page, are all long gone, unfortunately.
One of the more interesting remnants not related to Simtropolis is the old, long-defunct website from 2005. Most of these, of course, were never released, and a lot of them was just a picture of the location and a dream. There were a few that were released that weren't a part of the original 2005 list.
I remember I liked the idea because I enjoyed my 2010 trip to Michigan, and I liked the idea of some of the stores (Kmart, Meijer) being incorporated, as well as things I would've liked to see in my (idealized) city. I also liked the idea of a big downtown department store, which Hudson's would do well at.
I can understand why some of them were never released (beyond the obvious time and interest problem). Some things are straight-up too complex for SimCity 4, despite the ability to have large to-scale buildings, the way the city simulation works is that large entities (like airports, already not to-scale as is) have huge crime problems.
In any case, I don't have permission to re-host any BSP/mipro downloads (all at Simtropolis, though they allow guest downloads), so do check it out.