Mine is silly, but it would be to control time. To be able to "rewind" when needed (like when making a stupid mistake XD) or to be able to "pause" time when I want time to think things over and relax.
And if I hate how my life has turned out when I'm like 50, I could go back to being 20 or so and start over!
But, that's just wishful thinking XD I'm actually sorta happy with my life and I think that most of the things that we would usually wish for, we can work on ourselves. But it's so much easier to wish and dream... *dreams* ... ^_^