One reason I love the idea of a subway extension to Fisherman’s Wharf — and a stop in North Beach in particular — is that it offers a vision of the modern American landscape where big ideas, good ones, can come into focus with relative speed.

This isn’t the norm in our society, with its emphasis on studies and more studies, lawsuits clustered close behind. The legitimate need for scrutiny often tips to overzealous extremes — better to self-righteously say no than roll the dice on infrastructure changes at a major scale.

That’s why it is tantalizing to imagine a subway station across from Washington Square, taking the place of what now is an empty lot with a hole in the ground where a sign reads, “WARNING MEN WORKING BELOW.”

Such a vision may be the stuff of dreams, but it’s also the subject of a voluminous tire-kicking released last week by the San Francisco’s Municipal Transportation Agency.

A cadre of city planners worked through 14 scenarios on how to extend the Central Subway from Chinatown, where the northernmost stop at Stockton and Washington streets is scheduled to open in 2019, to Fisherman’s Wharf and back. The costs would range from $367 million to $1.4 billion, and the number of riders using the enlarged T-Third line is projected to climb as much as 55 percent beyond the levels now anticipated.

Whatever the alternative — the most intriguing of which would be a one-way subway loop up Powell Street to Beach Street, take a left and then return to Washington Square via Columbus Avenue — the inspirational starting point is the fenced-off lot across from Washington Square that until last year held the unsightly and long-vacant Pagoda Palace Theater.

The Pagoda came down so that the tunnel-boring machines used to carve the holes for the new subway could emerge. The site is being leased from a landowner who already has the Planning Commission’s blessing for a 19-unit residential building.

Connect North Beach

But that plan dates back to 2008, when everyone except transportation engineers thought of the $1.6 billion, 1.7-mile Central Subway as an undertaking (literally) that connected Mission Bay to Chinatown and went no farther. The manifestation of its stub end makes plain a logic that’s hard to deny: North Beach should be connected to San Francisco’s light-rail network.

In a city where the cultural center of gravity has been shifting southward for the past generation, the dense and historically dynamic neighborhoods that drape Telegraph Hill shouldn’t be left to tourists. Nor should they be the sole domain of people who managed to get a foothold decades ago, whether through rent control or a hefty mortgage payment.

This point was made by neighborhood residents who see the long-term value of being tied to the larger city in as many ways as possible. By the time the two tunnel-boring machines were extracted this summer, planners were hard at work on the prosaically titled “T-Third — Phase 3 Concept Study.”

“It looks very detailed, but we had a lot of data we could pull up” to take a first whack at what-ifs, said Paul Bignardi, a transit planner with the city. “We’re providing information to show what could happen, answer a lot of questions about how things could work and ask questions about where to go next.”

Left to me, the extension would go no farther than Washington Square: Let two lines meet on a single track below the empty lot, open a station and send out a press release. But Bignardi says that such an option could still cost upward of $500 million in 2014 dollars. Run the one-way loop via Powell, Beach and Columbus and you connect with extra riders for only a bit more than twice the amount ($1.139 billion).

Bignardi is careful to play down expectations about whether or not such an extension should or could come to pass. The concept study’s main purpose is to see if light rail to Fisherman’s Wharf should be included in larger studies of Muni’s long-term expansion. To make the cut it would need to be high enough on a list of city alternatives. If this happens there’s the question of where the money would come from, assuming that John Boehner and Mitch McConnell aren’t looking for ways to shower San Francisco with cash.

Years in the making

“The environmental studies alone, if we were given a green light to start tomorrow, two years at least,” Bignardi said. “The engineering studies, at least two or three years after that.”

Duly noted. But there’s a compelling power to the idea of an extension that, if nothing else, would make the Central Subway seem less like a boondoggle and more of a factor in the shaping of tomorrow’s city. The empty lot of the Pagoda was a starting point for dreams. Let’s see if it can become a starting point of something real as well.

Place appears on Wednesday. John King is the San Francisco Chronicle’s urban design critic. E-mail: jking@sfchronicle.com Twitter: @johnkingsfchron