BERLIN, Sept 22 (Reuters) - Legalising cannabis is about the only thing Germany"s Free Democrats (FDP) could easily agree with the centre-left Social Democrats and Greens, the FDP leader has said, sounding cool on the possibility of forming a so-called "traffic light" coalition. Christian Lindner wants his business-friendly FDP to be kingmakers after Germany"s national election on Sunday, at which the future course of Europe"s largest economy is at stake after 16 years of steady, centre-right leadership under Angela Merkel. In power since 2005, she plans to step down after the vote. Opinion polls show a coalition of the centre-left Social Democrats (SPD) and Greens with the FDP, dubbed a traffic light alliance due to their party colours of red, green and yellow, is a real arithmetical possibility after the election. But when asked by the Augsburger Allgemeine newspaper in an interview what could be easier for the FDP to achieve with the Social Democrats (SPD) and Greens than with Merkel"s conservatives, to whom he is closer, Lindner simply replied: "The legalization of cannabis." Asked to name any other issues, he responded: "I can"t think of many right now." Lindner, whose party believes in tax cuts and legalising cannabis, said he was unsure what the Social Democrat"s chancellor candidate, Olaf Scholz, stood for. "I"m not sure what his own political position is," he said. Scholz"s SPD saw its lead over Merkel"s conservatives narrow in a poll published on Tuesday, pointing to a tightening race just five days before the election.