Which countries can or should come together into one country and which regions within a country are better off separating?

Instinctively, I think countries that share the same language and economic system could potentially come together and spend less in bureaucracy. This thinking would group some Latin American countries together. On the other hand, some countries would benefit from such an arrangement while others will suffer. Some countries ended up becoming far better economically because of the separation. On the other hand, which regions are better off separating? ame