For "true Europeans" like Mr De Clercq, a Belgian schooled in the integrationist ideals of the six founding European Union members, such a prospect has been a long-held dream. Outside his window there were even signs that the grand construction was under way - cranes were lurching over a new parliament complex where Poles, Estonians and Slovenians could soon sit with the rest.
"With enlargement, at last European integration can become a continental project," mused Mr De Clercq, a former European commissioner.
Yet this federalist stalwart could not hide his deep foreboding. And along the corridors outside, others from the club of six - Germans, French, Luxembourgeois, Dutch and Italians - were also gloomy. Far from heralding a deeper union, Europe's integrationists realised last week that "sixism", as their type of federalism is sometimes called, is probably dead and that enlargement heralds a dilution, a stretching and a thinning in every direction.
The problem, they say, is that member states have failed to strengthen EU institutions. Only truly federal structures can withstand the pressures that expansion will bring, they believe. But at the Amsterdam summit in June, the governments refused to take even the smallest steps towards greater power-sharing.
"I fear that without these reforms first, enlargement may mean diluting Europe: if that is the case, we are lost," declared Mr De Clercq. Such fears have been stirred by the 43,000-word Agenda 2000, published by the European Commission, which details exactly how expansion should proceed.
The Commission suggested inviting five states from central and eastern Europe - Poland, Hungary, the Czech Republic, Estonia and Slovenia - as well as Cyprus, to join as a first step. A further five states - Lithuania, Latvia, Slovakia, Romania and Bulgaria - could join soon after.
The document attempts to be positive, talking of bigger markets and the dynamism of diversity. But never have the practical problems been laid so bare - and never has the likelihood of diluting the union been so evident.
Take the simple matter of everyday business. With a union of even 21 far less legislation will be passed, the Commission admits. In the Council of Ministers a simple tour de table already takes three hours, as each minister has a say. "How long will it take in future?" asked one Eurocrat. "They just won't be able to do much business - still less see the whites of the other man's eyes ... I mean, just think how large the Commission table will have to be."
It was not a frivolous point. The Commission acts by consensus and their table is always round. With many more commissioners, the risk is not merely that they won't understand each other, but that they won't even be able to hear.
The need to translate into an additional 11 languages, with as many as 4,000 extra translators coping with possibly 244 new language combinations, can only slow procedures down. And the new parliament building in Strasbourg, like the buildings in Brussels, is not even big enough to house such new armies.
Nothing makes dilution appear more inevitable than the Commission's proposal for funding expansion. The first wave would be carried out without increasing the overall contributions by member states. Optimistic predictions of growth and redirecting funds from existing EU programmes are expected to cover the cost. As one MEP said last week: "They're going to cut the same cake 26 different ways. So of course less can be done."
After expansion the union's population would grow to nearly 500 million, and the land area would expand by nearly half. Each applicant country brings with it large ethnic minorities - in Estonia alone 30 per cent of the population are Russian.
Expansion would bring 100 million new consumers, but with only a third of the purchasing power of existing EU consumers. In some countries as many as 30 per cent still work on the land (against an EU average of 5 per cent) and price differences for farm produce are staggering - up to 80 per cent more for tomatoes, and 40 per cent for milk and dairy products. Soviet-built nuclear power stations still provide 8 per cent of electricity in some applicant countries.
The commission has tried to pick the cherries off the east European tree. But Poland alone could shoot to pieces the carefully-designed "fortress Europe" policy aimed at keeping out unwanted immigrants and criminals. Poland, states the commission, is a transit route from Russia and Asia, yet has "no effective controls on migrants" who are "vulnerable to exploitation from criminal gangs".
The problems presented by Romania, where "the concept of hazardous waste has not been defined", or Latvia, where barely one in four have a telephone, are enough to send Brussels into a permanent spin.
Nobody will say they are against enlargement. That is taboo. Britain, of course, has always favoured it precisely because it would slow integration. But there is a solid bloc in the EU that is determined to stop the achievements of 40 years being diluted. Some countries are signalling they may try to delay the accession talks.
Meanwhile, MEPs are warning that unless member states reform the EU institutions to create a deeper federal structure before expansion happens, the parliament may veto the entire project as unworkable.
"That is the warning" says Mr De Clercq. "But will the European Parliament have the guts to do it?"