You already have Die Linke.
Lafontaine & Co never managed to turn Die Linke into a Left SocDem party. It has too much bagage and its very existence blocks the advance of a credible leftist alternative.
I was speaking of Jeremy Corbyn more specifically.
It remains to be seen if left-wing social democrats can appeal to a broader audience generally, anyway. Throw in the Greens and they're already at 20%. Outside of Greece, where has a party to the left of mainstream social democratic party done better than that? Even Podemos is polling in that range.
Most usually, such parties struggle to get over 10%. Their room for improval is questionable.
Maybe this is more idiosyncratic than you are looking for, but the Icelandic Pirates?
Needless to say, the amount of Greens who would be happy with a coalition with the CDU would seem to indicate a Linke-Green merger is a long way off. Unless the party underwent a Swiss style schism.
