It already happens. Look at the re-introduction programs for the grey wolf. This was probably decided by some jokers iving in Manhattan. Well, as predicted by those living cose to the land, the range of the wolves has spread, and is driving ranchers out West to distraction.
I know of a couple of recent spottings of panther in Michigan's UP, from first hand accounts ( panther haven't been seen up there for a long time ). The DNR in Michigan denies the sightings.
As far as human-free habitat, some of that comes from NIMBYs who decide that it would be wonderful to live in a planned subdivision out in the sticks ( 'getting back to nature, don't you know, with city water, electricity, natural gas, and cabe TV' ) without considering the fact that bears aren't cuddly, panthers don't eat Whiskers for their meals, and deer are nothing more than rats with big ears and long legs.
My point exactly.
I saw a timberwolf a few years ago driving back from the UP. And people had been sighting them off and on for years up there. Hadn't heard about the cats though. Supposedly some doofus accidentally let his russian boars get out of the fence of their game preserve and now they have boars too. Nasty buggers. For now the DNR isn't saying anything one way or the other about whether it's legal to shoot them on sight. So people hopefully are.