Urban planners love an organizing concept. Cities originally built their economies around financial districts, but over the years they have pinned their hopes on other specialized centers, like arts districts, enterprise zones, science centers, and festival marketplaces. Today, every ambitious city dreams of having an innovation district.

This month, Drexel University and its development partner, Brandywine Realty Trust, outlined plans to shape 14 acres on the west side of 30th Street Station into a conglomeration of offices, labs, apartments, shops, and landscaped public spaces where the Steve Jobses and Bill Gateses of the future can invent stuff and recreate together. The $3.5 billion undertaking, meant to be phased in over several decades, is being called Schuylkill Yards, after the rail yards on its northern edge.

By my count, that makes the third innovation district to be announced in Philadelphia in the last year, after the Science Center's new uCitySquare and the University of Pennsylvania's Pennovation campus in Gray's Ferry. Actually fourth, if you count Amtrak's long-range plan for a neighborhood over the actual rail yards - rolled out this week for 2050, but unlikely to happen anytime in this century.

The futuristic landscapes portrayed in the recent flood of renderings are dazzling, inspiring even, but how many of those glittering skyscrapers have the slightest chance of being realized in glass and steel? Not many.

You can count on a single hand the number of office towers erected in the city in the last 20 years. Even if you believe Philadelphia is on the cusp of a meds-and-eds-fueled expansion, it's going to take a long time before Philadelphia attracts enough companies to fill the millions of square feet in the proposed innovation zones. Unless a Google, Apple, or other large, out-of-town tenant can be persuaded to open a Philadelphia office, there will be no iconic skyscraper, predicts Glenn Blumenfeld, a tenant broker at Tactix. It's just too expensive to build high-rises.

What's more likely is that we will see some of the modest, and less costly, parts of these plans come to pass. The developers will busy themselves with improvements that give their innovation districts the feel of a real neighborhood. They'll do that by laying out new streets, marking their territory with signature parks, and renovating a few funky older structures. We can also expect a couple of new mid-rise office buildings and some apartments.

Pennovation and uCitySquare probably have a head start on wooing tenants for their districts, but Schuylkill Yards may have the edge when it comes to location.

Since Philadelphia's founding, its central business district has been creeping west along the Market Street spine, from the Delaware waterfront to its current location. Drexel's 14-acre site is the first stop across the Schuylkill from the high-rise office core. Already, two office towers, Cira and FMC, have established the nucleus of what could be a new downtown on the West Philadelphia side. Manifest destiny is on Schuylkill Yards' side.

That district also has the advantage of being bookended by a major regional transit hub and Drexel's campus. If high-speed rail ever happens, it will be quicker to get to midtown Manhattan by train from 30th Street Station than by subway from parts of Brooklyn. Meanwhile, Drexel is likely to be its own best tenant in Schuylkill Yards, as it will need new classrooms, research space, and housing.

Still, you might ask, why would the no-man's-land around 30th Street be more attractive to tech companies than the city's old-school office district, where the best restaurants and shops are only a block away?

For all their trendiness, innovation districts differ in form from legacy financial districts. Tech companies want buildings with lots of light, surrounded by cafes, parks, and other social spaces. These offices tend to be rougher and more horizontal than those in Center City's towers. The occupants like to be around like-minded tech companies, labs, and start-ups, according to a Brookings report on innovation districts. Most important, they want a real neighborhood with residents, where the sidewalks don't roll up at 5 p.m.

"Millennials crave authenticity," explains William Sharples, a principal at SHoP Architects, which produced the Schuylkill Yards master plan, together with Adriaan Geuze, of West 8 landscape architects. Sharples is something of an expert on innovation districts, having done plans for ones in Miami, Berkeley, downtown Detroit, and San Francisco's Mission District.

Of course, creating authenticity out of whole cloth is tricky, especially as innovation districts all seem to follow the same playbook. Much like Pennovation and uCitySquare, Schuylkill Yards will start by establishing a central public space. This fall, West 8 will start work on what's being called Drexel Square, a 1.3-acre park on a surface lot across from the train station. Drexel president John Fry says he is determined to replace the crumbling SEPTA transit entrance at the park's southeastern corner by the end of 2017, when Drexel Square is scheduled to open.

Also this fall, SHoP will begin renovating the former Bulletin newspaper building at 3100 Market, the last design by George Howe, famed creator of the PSFS Building. ShoP's clever but controversial design calls for draping a mesh screen over its east facade. The modern veil is intended to instantly rebrand the 1950s industrial structure, and allow digital projections, including movies, on the surface. SHoP envisions a rooftop garden or restaurant that would be open to the public. It's a tricky intervention, though, because the Bulletin building is such an architectural icon. Few realize, however, that the east facade was drastically remodeled, once before, in 1997.

The plan's boldest move, however, is the creation of a heavily landscaped esplanade on what is now JFK Boulevard, between the station and 32nd Street. West 8, which originated in the Netherlands, has designed a "woonerf," the Dutch term for a street shared equally by pedestrians, bicyclists, and slow-moving cars. The esplanade will have no curbs, just a flat plaza with broad sidewalks and a protected bike lane. The esplanade effectively drags the Drexel campus east, putting it nose to nose with Center City.

Such streets and parks serve as the equivalent of the software, providing the instructions for operating the hardware - i.e., the buildings - that will come later. It could be 10 or 20 or 30 years in the future, but whenever the tech hordes arrive, the Schuylkill Yards innovation district should have a good framework waiting for them.