This is another of those threads where we try to decide what God can or cannot do, if he exists. Theists can then ponder the consequences of our collective decision.

We all agree that reality is independent of the mind of the observer. A real thing is to be found outside the mind of whoever perceives it. Reality would have rules which you could not alter or bend. For example, I cannot cause a past event to repeat itself, or delete any event from history/time/memory. This is because these things are real and independent of me.

Would you say that anything is real in that way for God? Or is everything a product of his imagination?

If he has a reality, there are a few implications for his omnipotence, at the very least. If he imagines everything, including you and I, then none of us are real.

Contributions?

One implication of our being imagined by God is that every single one of the thoughts which you think are yours are actually God's. If this is true, I can authoritatively tell you right now that God may be having a headache.