Why not follow the "One country, two systems " formula for China/HongKong as envisioned by Deng Xiaoping ?
In this case, one union, many systems. There should be three monetary systems, Northern Euro, Southern Euro and a Slavic Euro and then move towards convergence via rigorous identifiable metrics.
In this case, one union, many systems. There should be three monetary systems, Northern Euro, Southern Euro and a Slavic Euro and then move towards convergence via rigorous identifiable metrics.