I’ll be quite open and say I don’t know for sure, and am just going to speculate.
Firstly any European superpower is going to be nothing like the EU of today.
It is obvious that the governmental structure and economic structure of the EU is not compatible with superpower status.
Also, the governmental structure and economic structure of the internal EU nations is, mostly speaking, no compatible with superpower status.
If you combine Italy, France, Germany, Spain, you get a pretty good-sized population and a nice sized economy. But the people in those countries have not nor have they been for a long time willing to spend as much on defense as the United States does.
None of these countries has any global force-projection networks even remotely comparable to the U.S. Armed Forces (compare the force-projection of the U.S. Navy to any other in the world.)
A European superpower is possible but it is a long long way off. It’s going to involve a fundamental change in how individual European countries think of themselves and of the world, and a fundamental change in how Europe works together, because the EU just isn’t the template for a superpower. Though it may be a very small first step.
Ultimately I tend to think the next superpower to come up will be China. But it’s hard to say what happens with China because China has a 2,000 year history of caring a lot about being regional hegemon but not caring much about expanding any further than that.
The fundamental idea throughout the Age of Imperialism up until WWII was that if any one power was able to unify Europe, the world would follow. But that logic doesn’t really apply anymore.
We’re probably heading to a multipolar world.
The U.S. and China will be the big boys for a long time.
But there will also be very significant players on other tiers.
I sort of see it going like this in say 40 years:
1st Tier Nations: U.S., China
2nd Tier Nations: Some sort of union in Europe
3rd Tier Nations: Russia, India, possibly Japan.
All those nations will be significant powers capable of interacting with each other.
While I say we will be soon moving to an era of two-superpowers once again, I think we are going to be in a more multi-polar international system where two of the poles just happen to be really powerful.