I think the European concept is very powerful and very successful. At least it provided peace and wealth for a big part of Europe.
However, Europe has three challenges:
First, the integration of new members. Europe managed to integrate poorer countries like Greece, Portugal and Spain. It was quite difficulty, but finally we made it. Integration of the new members is still going on. Nowbody knows the problems that the integration of Turkey would bring. I thibk the people in the EU need to know what will be the final borders of the EU.

Second, this bigger Europe needs a new constitution. I think that is obviously. And it is more complex than it was before. There are countries that will proceed and others that will keep their independencies. Today the more concerned ones slow everything down.

Third, this is the position of Europe in the world. You see that Europe cannot even stop the US from war. There has to be more integration with joined defence and foreign politics.

Anyway. I am sure that Europe will overcome these probs. Maybe there will be a Europe of three degrees:
1. The main Europe with nations that want to form a new United States of Europe.
2. Others that do not want to do that step but still are members.
3. Associated countries with the same trade rights.