After completing v2 and after fixing known bugs what I want the most is Steam integration. Which I think should be a paid upgrade. Linux should also not bee too difficult as it supports OpenGL. If AppGameKit is going to be a popular indie software then I think these are important. There are also features missing for Android and iOS.
I don't know anything about the Windows Phone 8 market. I only know one person that actually owns one. A few have iPhones and almost everybody around here got Android phones. Since it isn't OpenGL it will take a lot of time for TGC to implement. And I wonder how important this platform really is.