Narnia is a clear metaphor.
The Shack ain’t.
I do not have much problem with people who consider what God can do. I have difficulty with people who attempt to put God in a box, and say what he cannot do.
God, in the Bible, never shows himself as a human (other than Jesus), as far as I recall. But he manifests in other forms (burning bush, Giant Hand, writing on wall). Could he manifest as a woman, or a set of three people? Could he do so in a vision?
I am unwilling to say that such would be impossible for God to do.
Exactly the point of the book.
I think God's hobby is to shatter pre-conceived notions about him and his nature.
It is completely possible to present to us - humans any form he wants to and sometimes to shatter our illusions about what he should be or not.
Keep in mind:
He invented the whole concept of male and female. Do you think he can't control his representation to suit our needs?
He invented the entire concept of TIME/SPACE. Do you think he is not the master of it?
Not a single leaf falls in the woods without his hand. Even in space where there is literally nothing, he is there.
We can't even begin to imagine how he can present himself. Your imagination is not broad enough to fill that space.
He amazes me every day with his beauty and grace. Awesome power, patience with me and all of us. I wouldn't want to be in His judgment but since he died for me, I don't have to.
It was a gift given from need before we even knew we needed it.
God's currency is nothing like punishment or death. He deals in life and love. All is for us out of Love.
Even tough love. His heart is broken daily because of us.
He had a plan from the beginning, knowing we would screw things up. - Because he designed us that way!
Let that one sink in.
He identifies himself as a male, not a trans.