2. Romana
Im really betraying my affection for the Time Lords here, but Romana was literally the one who got away, for both the Doctor and the Time War. She is the only other Time Lord in the series that has a plausible excuse for surviving the destruction of all Time Lords in the universe namely, she wasnt
there at the time. Though RTD claimed Romana made her way back through the gate and became President of Gallifrey but hey, it aint canon until weve seen it onscreen. In the new series, the only thing weve seen of Time Lords outside of the Doctor is the bad ones the Master and Rassilon, to be specific. The contrast of the tough, intelligent and moral Romana to the murderous insanity of other Time Lords would be refreshing, and her youthfulness and poise a changeup compared to the saucy flirting and innuendos of River Song. Not to say Romana cant be sexy and funny when she wants to, but the restrictions placed on the Classic Series meant that the subtext between her and the Fourth Doctor remained just that, subtext. I reckon wed see a more serious Romana, traumatized by the loss of Gallifrey but willing to smack the Doctor in the head when he steps out of line and do whats right by the laws of time an aspect of Mary Tamms Romana I, rather than the usual fan-favourite of Lalla Wards Romana II. Instead of being a reminder of the Time War and awful times for the Doctor, she would be a reminder of Gallifrey and his family and childhood, something we need to see in the increasingly alien character. And maybe we could finally get a canon answer to all that sexual tension between her and the Fourth Doctor. Preferably one that doesnt end with their divorce, like Ward and Bakers actually did, after two years of marriage.