Hello Earnest,
You said...
"What does it mean ? What do you think it means ? Does it mean I and the Father are one God ? Or I and the Father are one in nature ? Or I and the Father are one in substance ? Or I and the Father are one in will/purpose ? Or I and the Father are one in mind ? Or...whatever."
I believe to answer this question we, of course, have to take the verse in context.
John 10:25-33 "Jesus answered them, I told you, and ye believed not: the works that I do in my Father's name, they bear witness of me. But ye believe not, because ye are not of my sheep, as I said unto you. My sheep hear my voice, and I know them, and they follow me: And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand. My Father, which gave them me, is greater than all; and no man is able to pluck them out of my Father's hand. I and my Father are one. Then the Jews took up stones again to stone him. Jesus answered them, Many good works have I shewed you from my Father; for which of those works do ye stone me? The Jews answered him, saying, For a good work we stone thee not; but for blasphemy; and because that thou, being a man, makest thyself God."
By reading this verse in context, we see that this is not talking about "one in will". Jesus really meant that He and His Father were one.
"Does it mean I and the Father are one God?" Yes, I believe so.
Notice that Jesus says "my sheep hear my voice...I give unto them enternal life...neither shall any man pluck them out of my hand." Now, His sheep are in His hand, right?
Then Jesus identifies the sheep as being in the Father's hand. "No man is able to pluck them out of my Father's hand." So, who's hand are the sheep in, the Father's or Christ's? The answer is both. This is completely validated by what Jesus says next, "I and my Father are one." Taking this in context, I can't see how someone comes up with "one in will."
This interpretation is validated even further by the Jews reaction -- they knew what Jesus was saying. "For a good work we stone thee not; but for blasphemy; and because that thou, being a man, makest thyself God." And they attempted to stone Him.