The proper way to do this would be to make self of type This. But unfortunately this makes this to be of type Traversable[A]. Since Traversable is a subtype of TraversableTemplate, all methods of this are taken from Traversable. In particular the newBuilder method is taken from Traversable, which means it yields a Traversable[A] instead of a This.

The right way out of this is to change Scala's member selection rules, so that always the most specific type will be selected, no matter whether a member is abstract or concrete. I tried to fake this by having a method thisTemplate which returns this at the Template type. But unfortunately that does not work, because we need to call newBuilder on this at the Template type (so that we get back a This) and newBuilder has to be a protected[this] because of variance.
The less appealing alternative is implemented now: Forget the self type and introduce a thisCollection which is this seen as an instance of This. We should go back to this once we have ameliorated Scala's member selection rules.