Idea for Cortana.
I've seen a lot of people saying things they are going to name Cortana "Jarvis" and give it a male voice with a British accent. My thought was, why not make "celebrity" computer assistants a thing you can download. Get the official Jarvis from Marvel by paying $9.99, making the voice Paul Bettany (or a great sound alike), and changing the circle to an icon more clearly Iron Man themed. Or give us a Halo pack that had Cortana themed animations, Jen Taylor's voice (or a sound alike, assuming this isn't already what Cortana sounds like), as well as 343 Guilty Spark voice and animations.
Windows Phone is already about taking the concept of a smartphone and making it personal; making the digital assistant something you actively choose would go a long way toward making it seem individualized, and would gain a lot of attention and consumer interest.
I'm not saying this should be ready by launch, but ready for the next update to Windows Phone I think is feasible.