Greens have said that they're willing to join government if Merkel prioritizes climate change, shuns 'anti European populism' and works for 'a fairer society. I really don't see that being a stretch for Mrs Merkel.
The SPD have confirmed that they will not take part in another 'Grand Coalition' with the CDU and will be going into opposition. Neither the FPD nor the Greens can give Merkel a majority by themselves, she has to get them both and they don't necessarily get along.
If the SPD really have got only 20.0 percent of the vote it will be their worst result since 1933 and their second worst result since 1890. After such a drubbing I imagine that the SPD may want to break the coalition with Merkel. Merkel must either link with FPD+Greens or form a minority government