So then your problem is that access to the FTA markets comes with barriers to the non-FTA markets.
Then you’ve got a network effect problem. Let’s say one country declares it’s the new FTA starter. What does the second country’s situation look like then?
If the second country joins the FTA, then its effects will be:
Trade access to the small FTA market
Trade barriers to the non-FTA market
What you did with the opt-in idea is found a solution for some of the incentive barriers to this agreement. Sort of an incremental growth approach which is more likely to succeed than an all-or-nothing approach.
But then the rule that it involves tarrifs against non-FTA countries means there is a downside to it. Suddenly the utility graph has a big zone that’s below zero.
This is a really hard problem (one that is historically solved by armies forcibly consolidating territory into unified political units), and I hope you keep working on it.