Multiple unions within Europe?

After the British decision to leave the EU, the remaining EU Member States entered a period of reflection upon the future of the Union. There seems to be a new trend towards more regional cooperation, creating several blocks of European Member States working more closely together within the Union. Various Southern European states seem to be willing to form a strategic alliance together; the idea of a North Sea Union is also being revived; and furthermore the four like-minded Visegrad countries are jointly pushing for reforms of the EU.