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).
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).
Both the SPD and Greens are against all of these ideas as far as I can remember at the moment, so there's hardly a realistic chance for any of these ideas to get pushed through by a very small coalition partner like the Left would be.
Nuclear power getting phased out is very much a consequence of the accident in Tschernobyl about ten years earlier, similar to how the black-yellow coalition accelerated the outphasing of nuclear power in response to the public reaction to the accident in Fukushima. But it's clear that the CDU likes the narrative of the Greens being to blame for the lack of energy resources when none of this would be a problem if the most recent black-red governments hadn't slowed down the expansion of renewable energy in the last several years.
Also, what I forgot to address yesterday is your statement that conservative votes account for roughly 50% in Germany. You only get to this figure by including votes for the AfD, but there's a difference between being conservative and being a nutjob. Furthermore, that a coalition without the CDU is possible now is owed to the CDU losing their votes from four years ago to left-leaning parties, not the FDP or AfD on the right side of the political spectrum because those two parties combined are almost exactly at the same value as four years ago. That strongly suggests that many of the people in the center see a need for change, because the SPD and CDU aren't far apart on the political spectrum; I would categorize their overall policies as center-left and center-right, respectively.
And lastly, Germany is infamous for its boneheaded federalism, so red-green-red in the government wouldn't have that much of an impact on the individual parts of Germany who do as they see fit anyway. Nobody is getting ignored when the CDU doesn't lead the country as a whole. Well, nobody of the reasonable people I should say; AfD followers are a different breed who have conspiracy theories and warped perceptions of pretty much everything, so life is tough for them as long as the AfD doesn't reach its Endsieg.