I would like know how the other thinks about the following proposal

Hereby I propose an operator, which allows you to add an extra field to any object, like you can do java script.

When adding a field it looks like (use of := operator)

obj..someNewField := "a value"

When setting the field it looks like (if field does not exist give error)

obj..someNewField = "an other value"

When getting the field it looks like

someVar = obj..someNewField

In JS this can be simply implemented, where this function is already available and the implementation in Java can be done, by adding a hashmap to the fantom Obj class.

You can use Dynamic Invoke

That is not the same, the dynamic invoke will delay the type checking to the runtime instead at compile time.

What I mean is that I can add field to a object, while it not defined in the class of the object.

You could do it by yourself:

class DynamicObj
{
  [Str:Obj?] map := [:]

  override Obj? trap(Str name, Obj?[]? args := null)
  {
    if (args == null) return map[name]
    return map[name] = args[0]
  }
}

obj->someNewField = "a value"
someVar = obj->someNewField

Dynamically (at runtime) I think the -> operator gives you much the flexibility you need. In fact we use this technique for most of our data oriented structures to treat dynamically typed record field access as a -> call.

For compile time stuff, I still think C# static extension methods are the right direction. Just not sure ready to bite that off yet.