Something like this?
private static Dictionary<Type,Func<object>> factory = new Dictionary<Type,Func<object>>{
{typeof(string), ()=> String.Empty },
{typeof(MyClass), ()=> new MyClass() },
};
public static T MyDefault<T>()
{
Type t = typeof(T);
Func<object> func = null;
if(factory.TryGetValue(t, out func))
return (T)func();
else
return default(T);
}