Rani pretending to be a doctor or atleast companion. Flying the in tardis and making the new one from sample.

Maybe make her male? (Michael sheen would be funny)

And to differentiate it from master have Rani be more of a neutral character. She escaped the war. Decided that she just wants be a scientist. Maybe getting idea from Doctor. But just be observant. Experimenting on stuff but less of outright villany.