(Best alternative to) Class inheritance in VBA

I grasp at the conceptual level what you want to do, but I think there are a couple of issues in your code example. Also, you need to decide if you want to truly emulate inheritance, or just get the job done in an easy way.

First, in your example, Property must be Get, Let, or Set. I think you need this:

Property Set o() as Selenium.WebDriver
   Set o = WebDr
End Property

Secondly, allowing the caller to retrieve the original object breaks the abstraction you are trying to create. That can work but exposes to the caller that you have the original object, plus a bunch of stuff you have added. Plus, from the caller's standpoint, there is no guarantee that operating on that object will continue to be reflected in the extended object. To truly replicate inheritance, you would have to code every property and method of the original, and not allow access to the underlying object. Otherwise the caller has to know when it's dealing with your extended class vs. the original class. If you hide the original object, then the caller does not have to go through the extra step of retrieving the original object, and operating on that. However, this requires writing a method and property for every method and property in the original class (which normally inheritance would do for you under the covers).

Property Get OriginalProperty()
   OriginalProperty = WebDr.OriginalProperty
End Property

Property Set OriginalProperty(NewOriginalPropertyValue) As Long
   WebDr.OriginalProperty = NewOriginalPropertyValue 
End Property

Sub OriginalMethod(SomeArgument As String)
   WebDr.OriginalMethod(SomeArgument)
End Sub

Further, as to your example, it is not clear how you can reference a non-existent method or property in the WebDr object:

Sub myExampleMethod1()
   WebDr.CustomizedSomething...
End Sub

I think you would need something like this to create a new method, which references available methods/properties:

Sub ForcePositive() ' Custom method (illustrates CustomedSomething with a concrete example)
   
   If WebDr.Property1 < 0 ' I do not have a spec for this class so I don't know the properties, Property1 is fake
      WebDr.Property1 = 0
   End If

End Sub

or something like this which creates a new property:

Private localNewProperty As Long ' declare at the top

Property Get NewProperty() As Long

   NewProperty = localNewProperty

End Property

Property Let NewProperty(NewPropertyNewValue As Long)

   localNewProperty = MyNewPropertyNewValue 

End Property

I have not used the Implements statement so I would have to study that.

I don't think there is any way to automatically check that your have implemented every property and method. Let me look into it and see what I can come up with.

However, the lack of a language mechanism for inheritance in VBA (much less multiple inheritance, or the Interface feature from Java) makes pure OOP very much a rompicoglioni.