It could be an incredible idea! But in this case, it will create a new block against Russia (except Belarus if Lukachenko stays and also Turkey which is playing on both side) and Putin, this chess player, will never let this happen!
Second point, for me, Turkey is mostly interested by Western Europe, not Eastern so this kind of block is not in its interest.
Economically such an union doesn't make sense. All these nations are economically dependent on Germany and forming such an union with non-EU member states such as Turkey and Ukraine will be impractical. A country can't be a member of both the EU and this new proposed union, because the EU laws doesn't permit any such overlap. And since the main purpose is to counter Russia, I don't think that countries such as Turkey and Bulgaria would show much interest. They are careful not to antagonize Putin.