God could have done it without Mary and still make Jesus fully human. He did it with Adam. Why not Christ?
Yes, in that sense it did not have to be absolutely positively Mary. She was a normal young girl, and simply chosen by God. However, I don't think it could have been "anyone" else because prophecy considerations still had to be met.
In addition, God COULD have "made a copy" of someone's DNA who was in the line of David, and then zapped that baby into Mary, but I just think that sounds less genuine. That also raises the "conceive" issue.