For that to happen, one of German mainstream parties would need to break away from the decades long tradition/unspoken agreement not to collaborate with the extreme right* which is unlikely to happen.
The 2016 US presidential election wasn't the first time Donald Trump ran for president, but his first successful run after two failed primary campaigns in 2000 and 2012.
Just about no one -- at least none of the mainstream authorities -- expected Trump to be elected president, ever, in 2012. I know I didn't. However, Trump was elected four years later.
The world is experiencing the sort of changes unseen since at least the end of the Cold War, if not World War II. Germany has certainly not been left unaffected.
I understand why you may dislike the notion of an AfD led government, but such a scenario should not be considered implausible at this juncture in history, especially should the German economy continue to struggle.
Hard times and disaffected peoples are what (alleged) political extremists grow on.
*That is not my classification of AfD but that's how they are viewed in Germany.
The NSDAP is banned in Germany.
If the AfD is indeed the NSDAP, then its members would not be allowed to be seated in the Bundestag.
Until that happens, the AfD are Nazis in the eyes of their many European criticis just as Trump is a fascist from the lens of his many American enemies.