Instead of going into the Time War, the Master stays and the drums are gone, though still having Skeletor issues. Instead of the Doctor regenerating to save Wilf, the Master beats him to it, way to make amends (at least start to anyway), beat the Doctor to saving the day, and the Doctor can't have more than 3 regenerations left, so if they're really the only two left she's not going to let him just go throwing his life away anymore, because she doesn't want to be the only one (addendum doctor has maybe 2 regenerations left (from master's pov probably would think 2- wouldn't know about metacrisis making it 1) master has 9). Regenerates into Missy (hence Missy and the Raggedy Doctor) will be protective of the Doctor because she wants to get better and he's her best friend, thank you very much, probably puts a tracker on him or something, because he's always slipping off and getting into trouble, but he wouldn't let her put a leash on him, so there's really only so much she can do, you know.