open source Windows Forms
Windows Forms is still the mostly used desktop technology for Windows Desktop Apps world wide, and WPF is only very slowly catching up. Still today, even the most new Desktop Applications projects become Windows Forms Apps, and on Windows CE industry devices, Windows Forms is often without alternative. Open Sourcing Windows Forms would allow the community to extend Windows Forms with functionality like true Mvvm support or seemless High Dpi scaling to find an easier, albeit long-term migration path to newer UI technologies, and it would increase business apps count for Windows 10, which derive from a new WinForms Developing paradigm that would most probable emerge.
Klaus Löffelmann commented
Well - the differrence is that Windows Forms is part of the .NET Framework, and many parts of that are changing to be available via NuGet and open source already. On top: Windows Forms is up to date in terms of 64bit, security, and much more. So, the resources Microsoft needed to open source Windows Forms would be much less, and on top Microsoft would benefit from it, because special interest business application had a clear migration path to Windows 10 over years to come. If you make VB6 available, what good is it for Microsoft, because the Apps would stick in an old paradigm or environment.
If Microsoft won't open source the VB6 programming language they are not likely to open source Windows Forms.
+1 - will be very useful
+1 for open sourcing whole Windows OS
Giacomo Stelluti Scala commented
I think that open sourcing whole Windows OSes will ben even better... It's now proven that switching proprietary products to open source doesn't affect security or lower earnings in any way. OS X is an example.