Well, I would like a WPI gauge (that is Wraps per inch), and a twist angle gauge, for starters. would be nice if there was a calculator to put in weight and yardage and get a YPP(yards per pound) too... and yes I could do that on a regular calc and do, but it would be nice to have a quick input calc for it.
I know the iSpin Toolkit has the wpi and twist angle stuff... not sure what else it has... but I do know there are people who would use this type of app, I'm not the only one.
Ultimately, in my dream app, I would also be able to take a pic of the yarn in progress and save it in a file to keep with notes on the wpi, twist angle, ypp, etc, plus notes on the dyer and or fiber prep process... but that is in my dream app. It woould basically make my phone a spinning journal.
I would be thrilled to see just the wpi and twist angle even, though... it would mean less things to carry when I go out spinning (which is pretty often, actually)