I love the Rani! And, my personal headcanon is that Rani Chandra IS the Rani, but she reverted herself to infancy and went through the Chameleon Arc to make herself human. Her parents were a couple who were going to have a baby, but she switched herself with the infant.
I think this is your most BRILLIANT cutie mark idea. It's time related for Time Lords, and it's a very complicated time device. That's the Rani in a nut shell. In her three appearances (including the 30th anniversary) Her schemes were more ridiculously complicated than the Master's or the Monks. It's so perfect, that you're a genius!