As I have written elsewhere, Merkel's main opponents are the members of the so-called Anden-Pakt, a power alliance now including three provincial PMs. One of these is Hessen's Roland Koch whom I mentioned in the defense minister's description, another (Christian Wulff, head of Lower Saxony) is the most popular politician at present (it seems to me, basically for looking good and not doing anything spectacular).

In the current setup, most CDU ministers are Merkel's followers. This could allow the Anden-Pakt guys to establish themselves as the shadow government of a virtual opposition, ready to take over after the next elections. That possibility can be realised if they can focus the public's eventual blame on the SPD and Merkel's circle rather than the CDU, and of course surviving as viable alternatives at hand.

Furthermore, I think it may be that it wasn't Merkel's decision to keep them out: they might have passed down offers from Merkel, hoping for something better after her. At any rate, they can limit Merkel's power through the second chamber of the German federal parliament (the Bundesrat), which consists of delegates of the state governments. And this analysis hasn't even considered what SPD ministers might be up to.

_{*Lunatic*, n.
One whose delusions are out of fashion.}