honesty, i'm going to say this will never happen. 1stly, the people living in the west already have problems with the animals that already live there, like wolves, bears, cougars, and coyotes.
if some environmental wackjob government passed a bill to reintroduce lions, cheetahs, and elephants into the american west, you can expect some serious outrage from the american people, as well as some choicely aimed bullets into some animals who would have been better off in africa or asia.
don't get me wrong, i love animals, but i personally don't want another thing that can eat me in my back yard.
also, a lot of people don't realize that the environment of north america has changed drastically over the past 10,000 years. the great plains used to be desserts, grasslands were far more extensive than the woodlands we have today. i really don't think that a re-wilding of the west will ever happen beyond a natural reserve or a park.