I'm sure that the 'core Europe' they would try to form would be: France, Germany, Benelux, and Italy (the 6 founding members) plus I think Spain and Portugal. I agree it'd be stillborn without all of them, and I don't think they'll be able to work something like that out unless the EU falls apart.
As I'm sure you know, a number of the European states are at a crossroads with regard to the transnational experiment, and it's not very clear how they're gonna play out. The expansion was a disaster IMHO, because it was governed by political rather than economic rationales. They will now pay the price..
Well The Netherlands is headed to a big fat no vote, and I think Italy is tired of France. This idea is so yesterday. There is a duty free zone, and it will remain. That was what the core was about. It had no other useful purpose.