Here follows very quick summaries of three modal theories which take God as central/fundamental (as befitting Classical Theism). All of them are ‘Actualist’ theories i.e. they take possibility as grounded in something actual.
 
Option 1 takes possibility to be grounded in the powers and dispositional properties of various beings. This is what might be termed the basic Aristotelian account. Note if we accept the highly plausible premise that the complete set of contingent beings could have been other than they actual are i.e there could have been different contingent beings, then in order for this to be possible said possibility has to be grounded in the powers of an actual being, and one which isn't contingent. This presents us with a nice modal Cosmological Argument for a very powerful if not omnipotent necessary being.
 
Option 2 takes possibility to be grounded in mental events and/or objects. This theory faces a number of problems, including the usual criticisms applying to all accounts which take logical objects to be mental. The theist is well situated to deal with one major criticism however: Conceivably the universe is not large enough to contain enough finite minds to serve as grounds for all possibilities; however if we were to turn to an infinite mind then that problem would disappear. Like option 1 it can be used to generate a theistic proof (in this case for a necessary omniscient being).
 
Option 3 takes the familiar Platonic account of Possible Worlds as Maximally Comprehensive Propositions or States-of-Affairs and adds the principle of Theistic Activism i.e. God necessarily creating all abstract objects in all possible worlds. This account is nice though suffers from the other drawback of Plantinga's account, that is that it leaves it disturbingly ambiguous as to what modal truthmakers actually are.
 
We can of course opt for hybrid models. Perhaps the Abstract Objects in No 3 come about (in a none temporal sense) as a result of the activity in No 2 which is in turn to be explained in No 1. I'll leave it to readers to consider how such proposals would relate to each theories individual problems e.g. the Bootstrapping Problem for Theistic Activism, as well as other fundamental tenets of Classical Theism such as Divine Simplicity.