Well there are a lot of half-baked ideas like:
unconditional basic income (which would cost quite a bit more than the whole state generates in taxes).
abolishing commuting allowance (even though it's not exclusive to cars and would cause people to settle even more in already crowded areas which in turn will cause an increase in housing prices)
Expropriation of housing corporations for billions instead of using that money to create public housing projects.
Withdrawal from NATO.
Immediate coal phase-out (even though the power generated from renewable energy sources are insufficient at the moment and that won't change in the upcoming years. Furthermore, it was the Greens "fault" (SPD as well) to phase-out of nuclear power in the early 2000s. Doing a 360 would cost dozens of billions and take several years until the reactors could substitute coal power plants. And a few years later they will be phased-out again because renewable energy has finally taken over.)
Well that were a few of the more radical ideas. Obviously some points aren't shared by the majority of a party but at least a good portion of it. Others are all too real and are either about to happen (expropriation) or part of a party's manifesto (NATO withdrawal).
It's kinda strange, that in an answer to Rol you put NATO withdrawal as a radical idea. Because Rol is from Austria, and Austria isn't a NATO member. Are Austrians radicals?
Well it was a requirement imposed by the Soviets in order to withdraw their troops from Austrian territory after World War II.
Were the Soviets radical?