There are two basic requirements for the icon. The first is that it should be easily adaptable to different teams, so I can change the text and/or colours to create a new icon for e.g. Manchester City. The other is that it shouldn't use any club crests, player images, kits, etc that might get the club's lawyers knocking on the door. Generic kits, footballs, etc are fine though. Other than those two things, anything goes!
This icon is for Windows Phone, not iOS or Android, so it needs to be square (no rounded corners) and shouldn't have the top-lit effect that most iOS icons have. It needs to be supplied in vector form and look reasonable at sizes between 99px and 480px.