With Berlin's entire air traffic set to relocate to the new Berlin-Brandenburg International (BBI) Airport in 2011, the future of the historic duo of airports in the German capital is hanging in the balance. With visions ranging from "Silicon Berlin" to Europe's biggest solar power plant, the cash-strapped city is characteristically open minded about the shape of things to come.

This week, Berlin's senator for urban development, Ingeborg Junge-Reyer, kicked off a brain-storming session for Tegel. Three-and-a-half years before its planned closure, she wants to drum up investors and protect jobs at Berlin's busiest air hub, which served more than 13 million passengers last year.

And at Wednesday's meeting, held just a stones' throw from the runway which was built during the 1948 Berlin Airlift, new ideas came thick and fast. Junge-Reyer told business and city officials the area had potential as a new trade center or a location for a new Berlin Olympic bid. Eric Schweitzer, president of Berlin's chamber of commerce, envisaged a new high-tech industry hub. Leading German architect Meinhard von Gerkan, who designed one of the wings of the airport four decades ago, said the 460 hectare site could be revamped into a solar research center and become a "mecca of modern sustainability."

His environmental aspirations for the airport chime with an unusual proposal for Tempelhof airport which were also made public this week. Roland Lipp, an engineer from the state of Brandenburg, is backing "Europe's first eco-city," an adventurous German-Russian plan featuring a 15-kilometer-long building (designed to house 50,000 residents), complete with roads built on rooftops in order to maximize green space. "Because of the streets on the roof, 40 percent of the area will be freed up for more greenery," Lipp said at a press conference held at a pizza restaurant. "But it won't turn into a new Berlin Wall: we are planning viewing holes in the building."

Investments of €2 billion have already been secured from two big Russian industrial firms, he said, declining to give names.

And any financially backed suggestions will come as relief to the government, which is saddled with a steep bill for maintaining the massive airport that was famously dubbed "the mother of all airports" by British architect Norman Foster. It costs some €9 million per year to maintain Tempelhof and, at present, the federal government foots most of the bill while about a one-third is paid for by the city of Berlin. Tempelhof is Europe's largest building and the bombastic proportions of its curved limestone facade reflect its Third Reich symbolism as a "Gateway to Europe." It was rebuilt in 1934 as part of a master plan to reshape Berlin for the Third Reich by Albert Speer, Hitler's much-admired architect.

But these days it stands as a broken relic: long stretches of corridors and huge hangars are empty and windows are boarded up. Its final flight takes off at the end of the month.

The building -- which measures the equivalent of 40 soccer pitches -- is listed and has tight restrictions on how it can be renovated and used. Plans mooted thus far, including a proposal to turn it into a luxury clinic, have all failed. Broadening their search for ideas, Berlin officials launched a "call for ideas" page on the Internet this week.

But, despite the latest round of audacious plans and optimistic visions, it's unlikely there will be a swift solution to Berlin's airport conundrum. That was all too clear to Meinhard von Gerkan who said time was needed to analyze the " formidable range of development possibilities." For such historic decisions, now is "no time for hasty plans," he said.