Deal should be offered also to Lithuania, Latvia, Estonia.
As you probably know, Poles and Lithuanians are not really friends today. Such offer, even if somehow appreciated by the Lithuanian government, would result in negative opinion in society. Especially, due to original idea of Polish "supremacy" in this... union.
Bulgaria, if Germany and Russia have consensus about it. Same goes for Romania but, it depend on tripartite deal of Russia, France and Germany. Finland, if Russia and Germany agree. Croatia is excluded if its up to Germany and ruling elite in Croatia that is deeply anti-Slavic but pro-Germanic.
Germany and Russia are excluded by the definition.
Belarus is big question. It may want deal with Międzymorze if it has neutral stance on other world powers (if is Międzymorze out of NATO).
Belarusians are divided between pro-Western and pro-Russian. Government is the part of the second.
Ukraine should be called into Międzymorze when finish negotiations with Russia about their new borders.
What "new borders"? O_o
i am sure this scenario is inevitable.
This scenario is abstract.
No one will join Intermarium of your dreams (it cannot be truly even named Intermarium), because:
1. They are afraid of results of leaving the EU;
2. Hatred between nations;
3. Alliance with Russians;
4. Archaic, panslavic ideas, without freedom and liberty.
No one will join such "union" - it would be like Soviet Union but without Russians and commie nonsense. Even I would be anti this idea, as I don't want any... genetic preservance in political cases. And the biggest thing: it has nothing to do with "Great Poland" you have mentioned.