Merkel should just resign now.
She's so worn out by now and doesn't even have her government under control anymore.
Add this to the fact of increasing corruption, incompetence and security-related scandals such as the BAMF one or the frequent killings and rapes by immigrants that she allowed in.
Her time is up. She's just limping along for the next 4 years because she's power-hungry and glued to her seat ... and it's painful to watch.
The odds are that Merkel, or whoever replaces her, isn't going to win the most seats in the next election anyways.
I'd bet on AfD picking up a narrow plurality of seats, and ultimately being able to form a minority government.