God's plan implies a working towards some goal, or a rhyme or reason at least. Karma implies no such thing; it simply describes an orderliness to the mental aspect of reality, in the same way that physics does for the physical aspect. Karma is in fact to be abandoned, or risen above, in the end, to the extent that one performs neither wholesome nor unwholesome karma as an enlightened being.
Simply put, Buddhism admits of no plan to samsara; it is utterly meaningless and to be risen above or abandoned in the end. Karma simply describes a part of the mechanics of the system and is not in fact all that important to Buddhist practice, in comparison to realization of the nature of reality as impermanent, suffering, and non-self.