Some theists give reductive accounts of such phenomena as morality and proper function in ways that involve contingent divine mental states. For instance, one might say that the proper function of x is to A if and only if God designed x to A, or that we ought to A if and only if God wills for us to A. Such reductive accounts are apt, however, to be in tension with divine simplicity.
Here is why. If divine simplicity is true, then God has no contingent purely intrinsic features. One way to argue for this claim is that if divine simplicity is true, then God is the truthmaker for all claims purely about God (this is the Oppy account of divine simplicity, further developed by Jeff Brower and myself). Thus if God has a purely intrinisic feature F, then God is the truthmaker of the claim that God has F, and hence the existence of God entails that God has F, and hence God has F essentially. Another way is with the intuitive argument that if God has a feature (think of it as trope-like rather than as universal-like) F and F is accidental, then F cannot be identical with God's essence (since F and the essence have different modal properties—the essence exists in all world where God exists while F doesn't). But that is contrary to divine simplicity.
Now, plausibly, if two worlds do not differ with respect to the purely intrinsic features of x, but the worlds do in fact differ, then they must differ with respect to something extrinsic to x. Therefore, if God has no contingent purely intrinsic features, and God exists necessarily, then any two worlds that differ, must differ in respect of something extrinsic to God. Thus, all contingent facts supervene on created reality, i.e., on that which is extrinsic to God.
In particular, this means that there cannot be two worlds where the same created stuff exists, and the only differences are in God's intrinisc mental states. In fact, it is impossible for there to be intrinsic contingent mental states in God.
This does not destroy the possibility of the theistic reductionist accounts of proper function and morality. But it does mean that these accounts cannot make the contingent divine mental states be independent of what is, in fact, in creation. On the contrary, these mental states have to supervene modally on creation. But if so, then there are certain features of creation such that, necessarily, x has function F or y ought to A if and only if these features obtain. And if there are such features, why not analyze the function or the ought directly in terms of these features?
(This does not mean that God is left out. For these features are in creation, and God's creative and concurrent causality is involved in them.)