Although the map is from mid-January 2017, it was circulated in real estate circles at least into the summer last year, suggesting that the plans are still relatively fresh.

Crain's first reported on one of the towers last week. Four sources, speaking under the condition of anonymity, said a high-rise residential building with more than 20 stories was planned to overlook Comerica Park east of Woodward. A specific location was not identified at that time.

According to the map, the Adams Tower would be on an Ilitch-owned surface parking lot and effectively envelope the Grand Valley State University building — blocking its Comerica Park view — next to the Detroit Athletic Club. A source briefed on the plans said the new tower would have baseball stadium-style seating for residents on its rooftop. The Montcalm Tower would be at Woodward and Montcalm Street, next to St. John's Church. The towers' planned heights are not revealed.

The map does not identify specific residential locations, although at least one of the two towers would be residential; the other could likely be residential or office, based on recent downtown development trends.

Robert Gibbs, principal of Birmingham-based Gibbs Planning Group, said the towers shouldn't be office space "because office goes dark and weakens at 5 p.m."

"It's a good idea to propose more high-density residential, more towers; the skyline can handle it," Gibbs said.

Two key hurdles for the towers and other developments to clear would be securing financing, which is tricky in a greater downtown market often reliant on gap financing to make up the difference between construction loans and developer equity, and demand for space, whether it's residential or office.

Can the market support the influx of the thousands of new residential units planned downtown and in its surrounding neighborhoods over the next several years? Although large blocks of Class A office space are currently at a premium, are there enough new local and/or national tenants to support it?

But if built, they would be part of a series of new buildings in the works that would dramatically redefine Detroit's skyline.

Among them: Dan Gilbert's 800-foot residential tower, the tallest building in the city and state, under construction on the site of the former J.L. Hudson's department store on Woodward to the south of the Ilitch family's development zone, and his Monroe Blocks development, which is expected to include a 35-story office tower and a 26-story residential tower, among other buildings up to 15 stories. And if Gilbert gets development rights to the half-built Wayne County Consolidated Jail site, high-rises are widely expected to come out of the ground at Gratiot Avenue and I-375 at the foot of downtown.

Philadelphia developer David Grasso has been considering a tower of up to 35 stories at the southwest corner of Woodward and West Grand Boulevard, and the owners of the Crowne Plaza Downtown Detroit Riverfront hotel are planning a 28-story second tower that would rise about 338 feet, taller than the existing 25-story building.