That wouldn't be easy to do. Windows Phone users are a small percentage of YouTube users. So many people are already using YouTube that it would be difficult for any startup to attract users and content.
I say MS turns xbox video into a YouTube type of setup. They will instantly have every video of xbox games uploaded to xbox video and that means less up loads to YouTube. They can then offer up ad free videos and larger video uploads to businesses and set up deals with netflix, hula ect ect. Do all that while setting up free accounts for anyone that wants one and let it all play on any device. It won't be instance but I bet people move to it pretty fast. There are lots of articles on the Web about people being passed at Google and the BS they are pulling with lots of things like Gmail and YouTube.