Imagine two new skyscrapers as big and swaggering as Liberty Place. Then imagine those towers sprouting from a full-size urban shopping mall - and not some miniaturized retail sampler. Try to visualize Marriott's convention hotel hoisted onto the mall's broad shoulders. And picture all of it on dreary East Market Street, where sneaker stores now rule and Strawbridge & Clothier's grand flagship has been reduced to a retail wallflower.

It may be hard to envision any new skyscrapers rising in Philadelphia at the moment, just when it feels as if the banks will never lend another dime to a developer. But those towers are pretty much all Kevin M. Doyle can think about these days.

Doyle is the president of Trinity Capital Advisors, a real estate and investment company that, until now, has specialized in collecting choice suburban office parks. In 2006, he paid $90 million to lease a full block at 12th and Market Streets, considered Center City's last big redevelopment site. Since then, he's been investigating how to occupy its 4.4 acres.

Doyle started by hiring Blackney Hayes Architects to figure out how much stuff could be physically piled onto the block. The answer is 3.5 million square feet. That would make the project a third bigger than Doyle's favorite model of mixed-use development: the Time Warner complex, a pair of knife-edged glass towers on Manhattan's Columbus Circle.

It's too early to call the results of Blackney Hayes' explorations a plan, but it's sure not too soon for Philadelphia to fret about the details.

Doyle's project, easily the most ambitious private development in Philadelphia since Liberty Place, has the potential over the next decade to transform the downtrodden Cinderella of East Market Street into a sparkling white-collar office zone that mirrors its older sibling west of Broad Street. Even if Doyle realizes only select parts, it could clean up the retail mess along Market, which deters conventioneers from venturing down to Independence Mall. His proposed 1,200-room hotel would serve the expanded Convention Center.

At the moment, Blackney Hayes' scheme calls for leveling the entire block bounded by Market, Chestnut, 11th and 12th Streets. Doyle would replace the existing medley of commercial buildings - some good, some awful - with a single podium. The podium would enclose a shopping mall consisting of two or three levels. The structure would double as the platform for as many as four towers. All the parking would be underground.

But Doyle's ambitions go way beyond this one block, which he has on lease for 75 years from the city-controlled Girard Estate.

Blackney Hayes' study shows a covered arcade running mid-block through the mall, from Market to Chestnut Street. Rather than stop at the property line, the walkway would burst through confines of the mall and poke deep into the commercial district south of Chestnut.

This new pedestrian street would cut through the middle of the Chestnut Street block, slice across Sansom Street, and finally terminate on the north side of Walnut Street. At least a half-dozen structures would have to be demolished. Adjacent building owners also would have to agree to open their properties to the new walkway.

Doyle argues this north-south walkway is just the medicine to revive the tired district wedged between Broad Street and Thomas Jefferson University Hospital. But unless it is planned with a zealous commitment to urbanist principles and executed with surgical precision, the walkway could just as easily destroy one of the city's most authentic, textured places. It requires a developer of the new who also values the old, the shabby and the eccentric.

So far, the scheme sketched by Blackney Hayes amounts to a formulaic superblock development, found in downtowns around the country. Superblocks rarely make a city more interesting, even if they do concentrate density and activity. The retail spaces are nearly always clones, populated by the same chain stores. And too often, the podiums are disconnected from their surroundings.

To Doyle's credit, he has spent the last year visiting such projects, to learn from their mistakes. He rightly believes it's important to break the podium into bite-size retail pieces at street level, with many access points. The podium's architecture, he adds, should appear transparent and inviting, to emphasize connections with the city.

In conversations, Doyle says the right things about the need for high-quality architecture. He wants to hire different architects for the mall and each of the towers, so the massive development assumes the eclectic character of the neighborhood. He's currently interviewing designers at SOM, FXFowle and RTKL. They're all good firms that specialize in complex high-rises, but there is a whiff of the usual suspects in the list. What Philadelphia doesn't need are knockoffs.

Before he hires anyone, perhaps Doyle should spend more time talking with another developer - Tony Goldman, who transformed a tawdry stretch on nearby 13th Street into an elegant and exciting commercial ramble. Goldman, who is now negotiating with Doyle about a role in the Market Street project, succeeded because he recognized the beauty and integrity in what was already there. He brought out the area's latent strengths by renovating existing buildings, installing independent retailers, and repopulating the upper stories with offices and apartments.

Goldman has his doubts about the superblock configuration. A block-sized podium sets the tone for a by-the-numbers urban mall. Furthermore, Doyle will erase the area's remaining character by demolishing the Stephen Girard building on 12th Street, a 13-story tower from 1896 that is one of the city's earliest surviving skyscrapers. The current plan calls for a garage ramp there, directly across from the Loews Hotel's garage entrance. No one will walk down 12th Street again.

A better approach, Goldman suggests, is to break up the podium with real city streets. Doyle will still be able to build everything on his to-do list, but it would be configured in an organically urban way. "The whole approach now is suburban," Goldman explained. "Once you take pedestrians off the street, put them in a controlled environment, it starts to be too homogenous and not diverse."

The message is simply, "Keep it real." Real streets. Real shops. Real architecture. Only then will Doyle's development feel as if it belongs in a city as uniquely real as Philadelphia.

Contact architecture critic Inga Saffron at 215-854-2213 or isaffron@phillynews.com.