If Madonna hadn't done it a couple years ago an extended Dark Horse intro playing whilst people dressed as Ancient Egyptians dragged in a carriage holding a hidden Katy would've been flawless.
That and I Kissed a Girl are probably her best choice for opener, those are hits I imagine suiting a 30 second minimal instrumental introduction best and build up hype for her appearance. Damn Roar, it better at least be a shakeup from the tour introduction.
