Unterrigenated Inhumans.
Are there millions, thousands, or hundreds of them across the planet?
How easy is Terrigen to come by?
Sourcing pounds per year makes it more valuable than sourcing tons per hour.
Are the Kree going to show up with a few thousand diviners, or are they going to massdrive the planet from orbit converting the convertable and removing all other humans from play en totem?
If I had to guess, I'd say it'd be something to do with Attilan. Maybe it gets destroyed in the upper atmosphere and enough mist gets spread across the planet to trigger millions.
To put it in perspective as to how many potential Inhumans there could be; it's estimated that around 16 million people can trace their genetic heritage back to Genghis Khan. That's one individual that lived only 800 odd years ago. The Inhumans were presumably a sizeable group that lived several thousand, if not millions of years ago.
Even if the Inhumans largely kept to themselves, if just a handful bred back into the general population over the millennia, that's technically more than enough time for the "Inhuman gene" to spread to every human on the planet today. Indeed, our most recent common ancestor is calculated to have lived sometime between 100-200,000 years ago.