There are traditional titles, and then titles pertinent to ones tradition. If you are creating your own deck, use your own symbolism and titles- it will work better for you.
For example the tarot was repopularised by the Golden Dawn (Or at least Waite) in the early 1900s, and most decks follow that astrological order, symbolism, and numerical order.
Crowley changed this when he created his Thoth deck, changed the deigns, changed some names, changed the astrological and numerical order etc. as it fit with his tradition and personal thoughts on the matter after many years of research.
Lesson being, do not be afraid to be creative and apply some changes if you feel you want to. The title 'Mother' is an excellent fit for the Empress card.
The empress card is a symbolic form of the Hebrew letter Daleth, which means door. The esoteric interpretation of this is it is symbolic of the womb- the door to life, and thus the mother.