When city and business leaders imagine downtown St. Paul’s future, they conjure impressive hotels on the riverfront and near the Xcel Energy Center, a high-rent office tower rising above a light-rail station, more shops and clubs on Wabasha Street.

Dreamers? Maybe. But Mayor Chris Coleman and others insist there’s rarely been a better time for the capital city to capitalize on recent high-profile downtown projects — most of them driven by or subsidized with public dollars — in order to generate new private investment there.

On Friday, they unveiled a conceptual plan designed to help developers see the potential in four vacant downtown sites: across from the Xcel Center on W. 7th Street, in and around the former Macy’s on Wabasha, along the Mississippi River bluff on Kellogg Boulevard and at the new light-rail Central Station at 5th and Minnesota Streets.

“There is so much momentum in the city of St. Paul right now … but the fact of the matter is, we have to keep that momentum going,” Coleman said.

No hard and fast development scenarios, let alone private money, have yet been attached to any of the sites. The plan itself says that “it is unlikely that all sites could be feasibly developed at these levels.”

But officials said that interest from regional and national developers already is high, and Coleman suggested that investors better get going while the going is good. The “window of opportunity,” he said, is open now but perhaps not for long.

As proof, the mayor pointed to new apartments and a grocery store either open or soon to come, the renovated Union Depot and its expansion of transit options, the Saints ballpark slated to open next year, and light-rail transit service set to begin this summer.

The plan is “aspirational … but it does capture potential energy to create new iconographic places in downtown St. Paul,” he said.

Each of the sites is owned either by the city, Ramsey County or the quasi-public St. Paul Port Authority, giving public officials some leeway in helping direct their future use.

But Coleman emphasized that the plan simply proposed some ideas, and that the city was open to any offers that echo if not precisely mirror the vision set out for each site.

The plan, which emerged from a task force that Coleman announced in his State of the City address a year ago, envisions a mix of residential, office, retail and entertainment space for the four sites.

The population in downtown St. Paul has increased 28 percent in the last decade, more than the average in major U.S. markets. About 74,000 people work downtown, and 8,100 live there.

The thinking is that with more people returning to the urban core, downtown St. Paul’s increasing apartment stock will draw more residents, which in turn will result in more businesses and stores to serve them.

Other factors expected to boost downtown: potential bus rapid transit on Robert Street, the possibility of future streetcars on 7th Street and a proposed bike loop encircling the central business district.

Michael Langley, the CEO of Greater MSP who headed the task force with Ecolab CEO Doug Baker, said the group surveyed scores of business leaders, real estate brokers and members of the development community to weigh the possibilities and determine the best uses for each site.

The centerpiece of the task force’s work is a 32-page document titled “Prosper: Momentum is Building,” which features striking illustrations by BWBR Architects of St. Paul of how each site might look with the right blend of imagination and investment. It’s a sophisticated marketing tool designed to whet the appetites of developers.

The plan envisions a 650-room hotel across from the Xcel Center, along with a ballroom and public plaza. A strikingly modern 400-room hotel could rise above the river bluff, situated on a base consisting of housing and parking.

According to the plan, the Macy’s site could see new retail and entertainment venues along with a Class A office tower and boutique hotel. Central Station could become a signature urban square dominated by another office tower, housing and commercial space.

“We’re looking at a very balanced mix. … Mixed use is going to be the name of the game,” Langley said.

Ramsey County Commissioner Jim McDonough, who led the county’s Union Depot restoration project, said that the sites are ripe for development. For instance, he said, the county’s former jail and office buildings at the old West Publishing site are finally vacant. “It gets people thinking,” he said.