The first thing is to be very precise in what you want.
I thought you wanted a window manager now you mention a full blow GUI API.
Frankly if you want to ovehaul Pharo GUI API , personally I think hands down a Pharo QT project will be far more valuable.
Windows may have an overwhelming market of 90% of OS users but the same is not true for developers. Developers are far more attracted to other platforms like MacOS and Linux. MacOS especially has a very loyal and quite large developer crowd. So a WIN API solution will not be as attractive as cross platform QT.
QT not only work on Win, Mac and Linux , it also work inside Web pages , iOS and Android.�� If we take into account that mobile is the most rapidly growing market you can imagine why QT is so much loved by devs.
I also recommend to get your hands dirty and learn C. Yes there are easier solutions out there, much easier, but knowing C does not open a door it opens a multiverse of possibilities. Its a huge bonus to have in your coding arsenal. C and C++ may not be as loved as Pharo but its a necessary evil none the less.
There is also the .NET options which provided the native gui on widnows and recently it open sourced and gained support from Microsoft for other platforms. Basically M$ bought Xamarin , the company responsible for MONO , a .NET that can run on pretty much any platform with support for C#, Python, Delphi, J#, F# and many other langauges.
But personally I do not trust Microsoft.