OK 2 opposite ideas here.
1 Simms is a revisit of a previous regeneration so that we all think we might get a familiar face return for the doctor.
Or
2 and I like this; for Missy Simm hasn’t happened yet. And the bringing the master back to fight in the time war hasn’t happened yet because the time war simms refers to has yet to happen.
Also I assumed that Simm line give me kiss was because Missy was about kiss the doctor.
Also Missy is locked out of the TARDIS controls. Would Simms be locked out to?