If the airline ultimately opts to leave a giant vacancy in one of the city's most famous towers, it would have a dramatic impact on both the Loop and the neighborhood it picks.

It's an opportunistic move by United, says veteran tenant broker Bobby Goodman, who co-founded Truss, an online platform that helps companies find office space. "United has the luxury of being able to kind of watch the buzz that Amazon has created, and that process has forced developers with these sites to put more visual plans together and actually push the pedal to the metal and make Amazon feel comfortable that they can deliver," he said. "United absolutely has an opportunity to take advantage of that."

Sources say United officials have entertained pitches to move their Willis Tower headquarters and operations center to locations including Tribune Media's 30-acre site along the Chicago River between Grand and Chicago avenues, where the media company has rolled out its vision to create a mixed-use campus and where the Chicago Tribune's lease for an 854,000-square-foot printing plant runs through 2023.

Other sites that could be in the mix were also part of the city of Chicago's official bid for Amazon's HQ2. They include Lincoln Yards, the 53-acre mixed-use campus that developer Sterling Bay has proposed along the Chicago River between Lincoln Park and Bucktown, as well as a vacant, 62-acre South Branch site where developer Related Midwest wants to build a mixed-use campus it has called the 78.

"We are always looking at what is in the marketplace, but don't have any plans to make an announcement," said a United spokeswoman.

Many developers call it a long shot for United to leave Willis Tower because the building's owner, New York-based private-equity behemoth Blackstone Group, will take any necessary step to keep it. Blackstone, which paid $1.3 billion for the 110-story building in 2015 and has since borrowed $1.3 billion against the property to finance a $668 million renovation, will need to secure its biggest revenue-generating tenant if it wants to sell the building after it completes its overhaul. The renovation, which is designed to modernize the 45-year-old tower to keep pace with competition, is slated to be done in 2022.

Another factor complicating a United move is that uprooting its roughly 650,000-square-foot operations center from Willis Tower—in addition to its roughly 200,000-square-foot corporate headquarters it moved to the skyscraper in 2012—would come with a huge cost. Plus, Willis Tower has location on its side with proximity to Union Station, making it easily accessible from most of the Chicago area, a key talent recruitment tool for many of the thousands of United employees who work in the city.

But new office projects have already proven effective in getting big office users' attention. Salesforce has zeroed in on a proposed office tower at Wolf Point to drastically expand its Chicago presence, while BMO Harris Bank is in talks to consolidate its Chicago operations at a new 1.5 million-square-foot office tower that developer Riverside Investment & Development is planning at Union Station. CNA Financial relocated earlier this year to a new 35-story tower at 151 N. Franklin St. from its longtime home at 333 S. Wabash Ave.

Skyscrapers also put some limitations on large companies that prefer bigger floor plates, as opposed to spreading employees across many floors. United has space on 18 different floors at Willis Tower, according to real estate information company CoStar Group.

Moving a headquarters can be a powerful tool for companies seeking culture change, as Motorola and McDonald’s have demonstrated in recent years. United CEO Oscar Munoz could take a similar tack to forge his turnaround strategy and walk away from a deal done by the airline’s previous management.

It's enough to make the airline—which is among the most creditworthy tenants in the market and recently recommitted to the city by helping finance an $8.5 billion expansion of O'Hare International Airport—at least try to leverage the flurry of emerging local office options for its benefit.

It may be a while before the company makes any real move one way or the other, but commercial real estate stakeholders will be watching closely to see how it plays out.