Just play Out of the Tombs before emptying your deck with Ad Nauseam or a self-mill combo (like Cephlid Breakfast)
Both are respectable pieces in their own right, but the chance to have another way to end things through a big combat swing, or through lots of extra impact damage, is too hard to pass up
On the other hand, Nethroi does a great job of keeping you alive thanks to deathtouch and lifelink, buying time until you can reanimate your choice of infinite combo using its mutate trigger
In the case of Vanellope, she reduces the Strength of all opposing characters by 1, for one turn, every time you play another Princess character
Probably no surprise at the top
DOUBLE BLIND SPECULATION So, theres ten of my best guesses (though far from my only ones) as to which doubling cards Wizards might have slipped into Double Masters