AP/Elaine Thompson

• Many are making their Amazon HQ2 predictions.
• Still, no one can know for sure which one of the 20 cities still in competition will be chosen. 
• It could come down to what some experts are calling the "hidden variable" — whatever business Amazon is getting into next.

Amazon's new headquarters search has everyone paying attention.

Everyone from Wall Street analysts, to robots, to voters in Pennsylvania's 18th district has some kind of opinion on where Amazon will go — or where it should go. A high-profile project like this one, with a $5 billion investment and up to 50,000 jobs attached to it, is bound to get people talking.

But even extremely smart people can only see what's in front of them. The notoriously secretive e-commerce giant could still have a few tricks up its sleeve as it continues its search for an HQ2 destination.

While most predictions have focused on things like transportation, education, cost of living, worker pool, and proximity to power centers, there's no accounting for what we don't know, says Greg LeRoy, executive director of Good Jobs First, a nonprofit advocacy group focused on government accountability and economic development.

LeRoy, who has been following Amazon's HQ2 search from the beginning, says there's something missing from most people's bets on where Amazon will select. He calls it the "hidden variable."

"We don't know what Bezos is going to acquire next, or where he thinks the company is gonna grow the most next," LeRoy said to Business Insider. "If we did have that information, it might suggest particular places that have comparative advantages in those niches. But since we don't know, it's all a guessing game."

Whatever this hidden variable is, it could be the thing that would, all else being equal, swing Amazon's choice from one city to the next. It's Amazon's next big, secret project — one that hasn't been reported at all and might even be in the very earliest stages of development.

Think New York having the advantage as Amazon moves into advertising or finance, or Chicago having an advantage if Amazon wants to beef up its relationships with food suppliers. Chicago is home to a good number of food-producing companies' corporate headquarters.

Analyst Daniel Ives of GBH Insights agrees that the decision is huge for the company, as the new city would be "the foundation of the evolution of Amazon."

Ives thinks that the variable could be healthcare, for example.

We don't yet know the full scale of Amazon's healthcare plans, but we've gotten some hints that they could be big. To do something big, it would need expertise not just in a tech-heavy city, but in one with a mature and flourishing natural and medical science talent pool.

That could very well be the leg-up that Boston, with Harvard University and Massachusetts General, or Washington, DC, with Johns Hopkins, have on the competition.

Of course, Amazon already has a lot of employees in many of these cities, and it's grown its business into many different industries. It may not need a second headquarters to move into any field it hasn't already doubled down on — it could just open a large local office, like it's been doing in Boston, DC, and New York.

Still, it's something to keep in mind as the HQ2 speculation rages.

NOW WATCH: The rise and fall of Hooters Air — the airline that lost the 'breastaurant' $40 million

See Also:

• Toys R Us says a 'perfect storm' killed the toy chain — and it blames Amazon, Walmart, and Target
• Millennials have been accused of killing everything from napkins to beer — but here are the industries that started dying when baby boomers were their age
• The key reason why Amazon Prime won't steal customers from Costco

SEE ALSO: I ordered the same items from Amazon and Walmart to see which site does it better — and they both had major flaws