Get Teddy maybe, or another president, or some other authority to set up a national park or equivalent in the Deep Southern Bayous and we might get the Ivory Billed Woodpecker to survive (some say it still does, but in the modern world of digital imaging I doubt the sightings to be true).
There is a foundation now working on a replacement, but I do not think it impossible to have the American Chestnut survive, there just needs to be no introduction of the Asiatic variant which carried the blight that killed it (harder) or just have a weaker blight, or American Chestnut with greater immunity (slightly easier).
I want more tigers, how can we get more tigers. The subspecies used to be numerous, now there is only a handful, I wish we could gaze upon a living Caspian Tiger.