I wanted to get something out of the way before a more lengthy discussion about Germany.
Looking at every poll taken in Germany since the last election (at least, those listed on Wikipedia)
There are a grand total of 4 (four) polls during this time (out of 741) where the combined CDU+SPD vote was below 50%
In each case, the CDU vote was 30%, and SPD vote 19%, and all were from the same polling company, and over the same 4 week period in May and June of 2016.
In each case, the combined total was 49%.
Keep in mind the use of thresholds, and that even without thresholds, 49% of the vote will almost certainly mean 51% of the seats, as many parties will simply fail to win enough votes to qualify, threshold or not. The main opposition parties (the ones that would win seats) did not top a combined 45% during this period.
In short, the existing Grand Coalition has consistently shown it can be re-elected.
Meanwhile, a SPD-Left-Green coalition only has reached 47% 4 times, meaning it's unlikely they can win a combined majority (at least, so this point)
A CDU-FDP coalition has reached 48% a half dozen times, and 47% many more times; but all of these instances are prior to the start of 2016.
The only two other coalitions that, from time to time, can regularly take a majority, are a massive 4 party coalition of the SPD-Left-Green-FDP, and the very unlikely right-wing coalition of the CDU and AfD.
Remember too that in the existing chamber, the left already has a majority; but coalition negotiations failed.
Consider also, the German states.
In places where the SPD could have formed a Left coalition VS a Grand coalition (with the CDU), the SPD often chooses the CDU. Brandenburg, or Mecklenburg for example, or the various states where the Left failed to win enough seats to qualify and the SPD sat with the Greens.
Berlin and Thuringia serve as potential templates for an anti-CDU coalition.