The ability to blend the different .net languages in a the same sourcecode, through the use of a new # tag.

Just like vb.net has XML Literals.

 

#Language="vb.net"
Public Module Module1

 Sub Main()
Dim n1 As Double=2
Dim n2 As Double=3
Dim result = Addition(n1,n2)
 Console.WriteLine(result)
 Console.ReadKey()
 End Sub
#EndLanguage
 
#Language="c#"
 public  double Addition(double x, double y) {
 return x + y;
}
#EndLanguage

The c# is probably wrong (not my best) but you get the gist.

Developers coding in prefered language.

 

I don't think it be that hard to implement, all the required mechnisms are already built; the compilers. It may need a new linker.