I think the main reason I need the Rani to come back is because she always found the Master annoying even at the best of times 

and Missy is about 300% more annoying than any previous regeneration

it would be an absolute joy to see her flit around the Rani like “RANI MY DARLING YESS BAD GIRL TEAM UP” and for the Rani to be sitting there well and truly contemplating/planning her murder because she’s having tea parties on the counters in her lab and all she wants is for this idiotic menace to die already so she can finally have some peace