The Associated Press ran an article last week about Sergey Zimov and his attempts to "rewild" a small corner of Siberia:
Of his first herd [of Yakutian horses], Zimov said 15 were killed by wolves and bears, 12 died from eating wild hemlock that grows in the park, and two slipped through the perimeter and made their way back some 1,000 kilometers (600 miles) to their original pastures.
It's tough to manage these animals without acting as if they were domesticated. He's talking about bringing in bison from North America, of course most of these are extensively managed. It would probably be more realistic to pursue an economic model where the "megafauna" paid for its own management. Maybe that would get in the way of tigers and bears, though.