One would like to say it's just political debates, but this election is likely to be a bit different. So my humble prediction of the influences:
1. WikiLeaks. I know that the political folks will get all hyped up and say it's not possible, but I think in the heat of this campaign, it's very likely that correspondence between the SPD players, the Linke Party, the Greens, and state-run TV news people.....will occur. If WikiLeaks finds this and puts in the open?
2. ISIS hype? If you just had two or three marginal ISIS episodes to occur, it would an influence upon the election.
3. Trump and Steinmeier? If some criticism starts up and goes on for weeks and months...it'll eventually make Germans ask if Steinmeier intentionally started the issue just to hype some anti-Trump or anti-US sentiment.
4. Austria, France and Netherlands election. All three will an influence.
5. A BER delay? Most people expect the new Berlin Airport to have another delay. The thing is....if we get to some point in the summer of 2017 and there's some really negative news (like a 18-month delay now tossed onto this)....it'll be another negative for both the CDU and SPD.
6. BREXIT. What if the Brit Parliament has a vote and goes against the exit, and then triggers massive rallies and public anger? Could that flip some German voters to side with or against the riot crowd?
7. Russian involvement in the election? Could RT or some pro-Russia group hype up the slant on the SPD or the Linke Party? Or could they side with AfD?
8. Could Erdogan dump the treaty and allow 500,000 migrants to make their way to Greece and onto Germany?
That's the thing about this election.....there are lots of outside and unusual influences.