Shades of HAL/WIN32...
Well,
Back in the day, THAT was the platform we wrote to (and even on diff processors) and the "Win32" app that came out of the hind end of a compiler WAS a universal app...
Now, with so many different presentation UI's, what MS needs to do is get back to basics, and abstract as much of the programmatic support to a "universal OS layer", and THEN present programmers with a new kind of "Win32/64" GUI/Interop API set, with built-in provisos for different platforms (these could be compiler-driven and mostly hidden from the coder).
Forget the multitude of runtimes and "frameworks" for the first-and even second iteration, or they will mire the platform in a maze of confused dependencies.