Enlargement of the European Union will improve the foreign policy of the European Union. When you allow other countries to join in a confederation, that brings in a different set of views and beliefs than all the other countries, and that will diversify the foreign policy of the EU, which will lead to future success.
With enlargement of the EU, there will be more countries participating in debates at the EU. More participation means more points of view to be considered. When we consider more points of view, we can create plans and solutions that take more countries into account. When the EU enlarges, it is more capable of creating good solutions for more countries.
It's a cancer that kills culture, sovereignty and the economy of all it's members. There can't be a single law-creating entity for a continent as diverse as Europe. And the more it expands the worse it is for us living in it. Not to mention that it's sucking all the major EU economies dry.......
Enlarging the EU will damage the ability of the EU council to reach an agreed consensus, as more heads means more negotiation, which in turn will damage the ability of the EU to respond effectively and quickly to foreign policy issues, as one has seen from the Ukraine. That said, I don't consider that an argument against Enlargement, but rather one for a broader as well as a deeper union.