1. Despite all reaffirmation against it, Schröder forms a grand coalition. Merkel, as the loser of an election which was a few weeks before a sure win, will be overthrown by her internal enemies (of whom she has many: e.g. Friedrich Merz, Roland Koch, Edmund Stoiber). Her successor (maybe one of the above) will agree to play deputy in a grand coalition under sheriff Schröder, in order to prepare the real takeover for the next elections.
   
2. A red/red/green coalition. Highly unlikely, because of reasons (political and personal) I have already named.
   
3. Red/green, tolerated by the Left Party. Still unlikely, but not as unlikely as 2. A somewhat awkward model with no real participation for the LP in terms of cabinet posts etc., but with an informal consenus over the issues. The model has been successfully tested on regional state level in Magdeburg (Sachsen-Anhalt). But still, on the federal level, questions of the stability of this combination remain unanswered.
   
4. Traffic lights coalition. Let's forget this one quickly. I just can't see how this could possibly work.
   
5. Your version, whataboutbob. An interesting one indeed. A weakened Merkel, exposed to her internal enemies, possibly not able to hold grip of the party chair. The CDU/CSU having to find a new candidate quickly, turning the parties into turmoil. I'm waiting for a journalist who asks Schröder your question. I'd really like to hear the answer.

I have not come across any numbers predictiv this year's voter turnout, but I would guess it will be within the average of the last two decades (roughly 75-80 %). Fear of extremists (in Germany, understandably, first of all fear of rightist extremists), has been largely absent - no matter if high or low voter turnout. Which is, by the way, a success that has to be ascribed to the Left Party. (A topic I really want to elaborate on; maybe tomorrow)

is there something  like the GOTV in Germany?

Not like in, say, the US. Simply because there is not the necessity. In the US, you have weak party organisations and low voter turnout, so distinct GOTV actions and organisations become necessary. In Germany with its strong party organisations, GOTV happens along the line of party membership (party members bringing the family to the ballot box, making shuttle services for old people's homes etc). But it is not being called "get out the vote", and there is not a special word for it, at least as far as I know.

This isn't a landlside and is close by all means, but there is still a clear favorite.