For quite a few years, my answer was "nothing." I knew what I wanted from life, the key things that would make me happy, and I had them all.
Aging took the glory out of one of those things, the economy another, but it was absolutely awesome to have all of the things that I considered key to happiness.
The energy and ability to be at two places at once... or just a way to control time... BWAHAHAHAHAHAHAHAAAAAA~ (though I'd try to do good with that... then again... changing history can be good and bad...)