Tonguez
A suffusion of yellow
As a long-time fan, I know that there's a segment of the fandom that's wanted The Rani back for years, but I honestly don't think it's a good idea. They'd need to do something unique with her, and "cold, evil, scientist lady" really is one note.
I'll hold out some hope, but at this moment, I'd rather have Missy back.
The Rani is a very different character to the Master though, the Master is Charismatic but Cruel, she's a force of Chaos motivated by ego and obsession. Shes very much the 'evil Doctor'. The Rani was more cynical logic, she's motivated by control and knowledge, but she isnt overtly destructive. Morality is a distraction and the Rani dislikes the Doctor because he's an interfering idealist.
Of course a lot of why the Rani worked was because Kate O'Mara portrayed her so masterfully, OMara was a serious dramatic actress who had done lots of stage work including Lady Macbeth and Catherine De Medici and she brought the same gravitas, and theatrical flair to the Rani. Moreover Omara didn’t play the Rani as evil or mad but as cool, intelligent, and dangerous and never doubted that she could out think everyone in the room including both the Master and the Doctor.
Of course theres no guarantee that the new Rani will follow the same characterisation, but I hope she does, it would be nice to have another Time Lord antagonist who is the Doctors equal without being actively anti-Doctor