MASTER: No. Never. Missy! I will never stand with the Doctor!
MISSY: Yes, my dear, you will.

Rinse, repeat.*idiom*

However, I can understand that it would be challenging to one up the previous storyline for the same villain. Buffy had one Big Bad per season. Then it was done.

Maybe do something on the lines of that Surprise Banana Trick Eleven did in the episode: Lets Kill Hitler.