Process does not argue that God is imperfect or incomplete. Rather it argues that God creates an interactive cosmos that is open ended. Thus, it argues that God is open to new experiences as God interacts with creation and created beings.
Process Theology may not argue imperfection directly, nor make any such claims, but that is what the conclusion seems to be. God can interact with creation efficiently without having an open future, which renders the assumption that it is "open ended" an unnecessary conclusion.
We believe in a Triune God. God is one, while existent as three distinct persons: The Father, the Son, and the Holy Spirit. Without the universe, God is eternal and timeless since time began with the universe. The Father is eternal and timeless as are the Son and Holy Spirit. Once God wills to create, however, the Son becomes temporal as does the Spirit which is a thought actually grounded in Scripture (The Father created through the Son). And once the incarnation, the Son becomes temporal to a more fuller extent (physicality). Yet the Father was not incarnated, and so must have remained timeless as He did without the universe. So, while God the Father could be said to be timeless, it could also be maintained that through the Son God is also temporal and interacts directly with the world and the rest of creation.
Otherwise, how else can humans have free will or be responsible for their own actions? And, how can God answer prayers if God cannot, by creating a deterministic and pre-known system, be open to changes in interaction. Prayer makes no sense if God cannot be open to answering those prayers by planning and knowing all events past, present, and future.
For Process Theology to hold up, especially in regards to the concept of free will, it must be shown that free will and determinism are mutually exclusive, or else this is an unwarranted premise. IMO, there is no such valid argument, so free will and determinism are compatible. That means that even if God has foreknowledge of event x, I still am responsible for the happening of x and the consequences.
This is because free will and moral responsibility are basically one in the same: one cannot be held accountable unless he was free to act, or if one was not free to act, then he cannot be accountable. So, what is moral responsibility? In a nut shell it is the mental competence one has of comprehending their reasons and motives for acting, as well as understanding the consequences of those actions. Free will is then the actual ability one has to carry out those actions without any external or internal factor either forcing or preventing the one from acting as they wish.
So, long as there is no external or internal factor preventing or forcing me to partake in x, then I am free. As long as I understand my reasons for acting on x, I am free. The foreknowledge of God is obviously no internal factor forcing or preventing me to act, so the question is whether it is an external factor. Knowledge, even it being prior, is not a causal relation that could make an event happen. In other words, knowledge does not cause things to happen. Knowledge is to simply be consciously aware of something, so foreknowledge is the attribute of God meaning He is consciously aware of all things in the future, not necessarily that He causes or plans for all things to happen.
On top of all this, it would actually be consistent for Process thinkers to say there is such a thing as determinism, at least in the causal sense. Causal determinism is the idea that future happenings abide due to past circumstances or events. For example, that I am replying to this now was contingent on me turning the computer on, opening Firefox, typing in the CF web address, etc, etc. The actuality of me now replying wouldn't be an actuality if not one of those previous things did not happen. Think of this in relation to God and creation. God created the universe, man was in fellowship with God in the Garden, man fell, Christ was sent, and so on till where we are currently. See, if God had not created the universe, we wouldn't be here right now, and none of those other events would have happened, hence causal determinism.
God is the Living God. No where does the Bible say God is the "Static God" the "Unmoved Mover."
Likewise, no where does the Bible say God is "open," or much less "open to new experiences."
Being able to experience novelty does not mean God is not perfect, it just means God can experience novelty. Nothing more, nothing less. Also, process does a good job of explaining evil, disaster, tragedy, without impugning God as the creator and planner of that evil, which, if God knows all absolutely and planned the course of history, God would be responsible for. Now, that would be calling God imperfect as you would be attributing evil to God.
Experiencing novelty implies change. Change implies imperfection, as one either changes for the better or worse. I assume according to Process Theology, God changes for the better, since to experience novelty is something to be considered good. So, as God gains knowledge of the future as the future happens, he also changes from not knowing event x, to knowing event x. That is an internal change.
The Bible describes God as being perfect in knowledge, and that nothing escapes His knowledge. If the future is not known, then it cannot be said that God is perfect in knowledge, and thus not really perfect at all as something is missing is gained, namely the knowledge of the future.