I like the metaphor of God as Mother, it connotes warmth, gentleness, creation and unconditional love. Father connotes a slightly more distant relationship and a strong sense of discipline. Combining the two into one concept I think is a powerful notion.
However,
the Father and the son are presented in the bible as male and it tells us that God's order is for male to be above female in authority. It would not be consistent with God's plan therefore to represent God as female and have His authority brought into question. The Holy Spirit is also announced by Jesus as a male person.