The Real-time Programming C# WinForms Controls in VisualStudio's IDE (i.e. without using F5) blog entry shows how to use the VisualStudio C# REPL - O2 Platform extension to update/fix an existing WinForms control without going through the painful (and slow) process of VisualStudio GUI programming.
What you will see in that example is that I will dynamically compile and execute a WinForms Control, without using VisualStudio's Debug/Run capabilities to see the new changes in action. I was able to compile the modified Source Code file, and using reflection, was able to create a new instance of an WinForm's Control (without leaving the VisualStudio's IDE).
And the best part is that we can code our tests in an REPL environment (think Roslyn), which allows the support for complex (and unit-test like) scenarios.
I can't underestimate how powerful, fast and efficient it is to be able to have this type of REPL environment while coding.
This is what I would like to see on the next version of VisualStudio.