Make a managed class containing all APIs declarations
Instead of "Pinvoking" each time the functions
I think this would be better served by just properly integrating the unmanaged calls into the framework. A good example of this was Ping - in 1.0 you had to use P/Invoke, but in later releases it was built into the System.Net namespace. It makes more sense to do it that way than have one giant class containing all the DLL imports, structures, constants and enums.
Of course, best situation of all would be if Windows just scrapped Win32 and made .NET the actual API - maybe that's what WinRT will turn out to be in Windows 8?