I'd really like to see a total fusion between Windows Phone and Windows RT, taking the best of both worlds, with much more and much better apps variety, and all inside my Surface 2.
Playing devils advocate i say, be careful what you wish. Currently Windows RT is Windows compiled for ARM. So by this very fact the development of features goes in lockstep with Windows. This means as soon as a new feature is implemented, it is readily available for RT, as it uses the same sources. For this reason even the preview version of Windows RT8.1 was available the same day as the Windows 8.1 preview matching its brother feature by feature.
If you are trying to couple Windows RT sources to Windows Phone sources to any larger extend, you immediatly loose the property described above. In addition you make RT dependent on Windows Phone development cylcle, while Windows development runs ahead. In addition in between Windows and Windows Phone you open up a third (and separate) development branch, which would put a huge maintenance effort on Microsoft.
I assume what you really want is integrating the Windows Phone runtime environment into Windows, such that you gain app compatibility. A fusion like you describe it above is certainly the worst thing that could happen. There is not a single feature Windows (RT) would gain from fusion except app compatibility, which can be achieved much more conveniently by unifiying the runtime environment (and store), while still keeping RT as the ARM compiled build of Windows.