I think it would be incredibly useful to have an app that displays (on a live tile) my remaining data allowance. Right now, Data Sense just tells me how much data I have remaining for my current billing cycle. It would be nice if it would divide my remaining data by how many days I have left to give me my daily data allowance, so I can make it last without going over. Of course I can just pull up the calculator app and do it, but I had an app that did that on my Android, and it's a nice feature.
It'd be super easy to make a very basic app where you input your data allowance and when it resets, and have it divide the numbers and have the result displayed on a live tile to tell you how much you should use that day.