The outer limits – East, north, south

"The states of Europe at present are 57 in number." A conveniently coincidental figure. But not a 2007 prophecy for 50 years ahead. It is from the 1816 edition of the Encyclopaedia Britannica. It may take a while for the EU to get up to that number. But, as 50-year-fever reaches its peak, speculation is inevitably provoked on just where the borders of the EU will lie halfway through the 21st century.

Let us start with the current trend. Croatia, yes. Possibly the year after next. The other countries of the western Balkans, yes - and most of them within a decade from now. That could bring the EU up to 33 or 34 (depending on how far Kosovo has achieved independence) before 2020.

Turkey is a longer shot - not just because of the adaptations it still has to make itself, but more because of promised referenda in reluctant member states.

Ukraine - the other big beast at the EU door - might get in ahead of Turkey if it can shake off Moscow’s still-pervasive influence and can effect sustained reforms. Moldova could slip in on the coat-tails of its Romanian sponsor. A Lukashenko-free Belarus would also be a serious contender, allowing the EU to complete its east European collection.

Meanwhile, it would not take much of a wind of change to blow the low-hanging fruit of Norway and Iceland (plus Liechtenstein) into the EU basket - perhaps even tipping Switzerland out of its determined isolation. Optimists can thus easily predict an EU40 by 2040.

That is where the counting gets more difficult. The three south Caucasus countries are possibilities - Georgia is already bidding hard - and the EU’s energy-hunger is a strong argument for locking in such an important transit region. The same logic could ease open the EU doors even further along that corridor, to admit the energy sources themselves - bringing in as many as six of the ‘stans’ of central Asia.

But once energy leads the EU to expand as far as the borders of China, what logic could impede an embrace of other energy-rich states much closer in geography, history and political links? Through its neighbourhood policy, Europe has already invited the ten Mediterranean states of the Levant and north Africa to a cosier relationship. Council of Ministers Director-General Robert Cooper has recently penned a delicious future fantasy in which any country with Roman or Greek ruins would be considered European. And once headed down that road, Baghdad is only a short hop away (much closer to Athens or Bucharest than Brussels is). Then bringing Moscow on board could create a single market stretching as far as Japan and the US.

"Enough already!" realists cry. The EU is not the Eurovision song contest (up to a record 42 countries this year). Nor can it expand like the Champions League (UEFA’s membership now tops 55 - including Kazakhstan and Israel).

Even one of enlargement’s most avid supporters, Roman Shpek, Ukraine’s ambassador to the EU, knows there are limits: "Russia will never join the EU," he says, insisting that in any case the EU should never contemplate trade-offs between democracy and energy - which rules out north Africa too, in his view. Instead, underlining his country’s credentials as part of a Christian European tradition, he sees Ukraine within the EU in perhaps another 15 years. Slovakia’s Permanent Representative Maroö Šefc?ovic? agrees that the number of potential new members could exceed 40, but he sees only Croatia joining before 2014.

Not everyone is keen on defining the EU’s outer limits: "It’s always politics that defines boundaries, not geography," says European Parliament Vice-President Jan Marinus Wiersma. Former Parliament president Klaus Hänsch also resists drawing lines on the map: "It’s a question of political will and political ability to digest new members," he says.

Whether the future is predicated on principles or geographical positioning, it is a rash soothsayer who assumes linear development in a stable environment. A glance back over the last 50 years demonstrates how much the EU’s evolution has been driven by the unexpected. And nowadays, there are already plenty of internal and external menaces in evidence - to say nothing of what may be lurking in the shadows as Moscow and Brussels jockey increasingly for influence, and the emerging world’s economies overtake Europe’s.

Senior Commission adviser Jean Pisani-Ferry doubts that the EU will "muddle through" without major efforts to keep it going; he foresees that some member states will consider seceding and the EU will survive that test only if its systems are more robust. By contrast, the pressure for staying together is seen by Wiersma as increasing. In his view, consensus will be promoted because challenges such as tougher competition in a globalised economy, migration, crime, energy security, or climate change can be met more effectively by common action.

Further enlargements are likely in any case to necessitate the emergence of a different type of membership - akin perhaps to the ‘privileged partnership’ urged by European Parliament President Hans-Gert Pöttering and many of his fellow German politicians for Turkey. Management of an ever larger Union will become impossible without further developments in variable geometry for the EU. Former European Commission president Jacques Delors warned only days ago that the EU could unravel in 20 years unless it reforms its institutions.

There is no prospect of an EU superstate, but instead a multiple-choice pick-and-mix of policies, suggests Šefc?ovic?. He predicts that if the result is a large European economic area with different spheres of co-operation, "this would be the best contribution the EU can make to this century - Europe united in prosperity and stability, but more diverse". Perhaps the most accurate prediction for 2057 is 57 - not member states, but varieties.

• Peter O’Donnell is a freelance journalist based in Brussels.

"The states of Europe at present are 57 in number." A conveniently coincidental figure. But not a 2007 prophecy for 50 years ahead. It is from the 1816 edition of the Encyclopaedia Britannica. It may take a while for the EU to get up to that number. But, as 50-year-fever reaches its peak, speculation is inevitably provoked on just where the borders of the EU will lie halfway through the 21st century.