Re: An "add on" to a Windows phone?
The problem with that for the time being is that apps on Windows Phone don't necessarily appear on Windows proper and vice-versa. That may change with Windows 10, what with the rumours that it'll be the same thing on all the platforms, and I hope it does - Windows Phone *does* have an iPlayer app, for example, but Windows 8 does not.
It would certainly be nice to see some sort of iPhone/iPad or Android phone/Android tablet style coherence between the two so that apps you purchase on one device become available on all, regardless of form factor. Perhaps the more mature app store of Windows Phone could give Windows 8, or Windows 10 as it'll soon be, something of a leg-up.