So much happened in the season finale of 'Doctor Who' that will be the spark not only for further discussions but also for possible fanfic - the team-up of Clara and Ashildr/Me in their own stolen TARDIS, especially. But for me, the fixation is on the return of Rassilon, once the supreme leader of Gallifrey....
From the TARDIS Data Core:
I think the Grand Moff has set up Rassilon to return yet again to the series, perhaps as a recurring villain to add to the Doctor's gallery of rogues. The show can't keep returning to the Master/Missy as the only individual nemesis. Rassilon would definitely have a score to settle with the Doctor after his banishment.
If he was free to come back, I think Donald Sumpter should get another shot at playing the former President. Since Rassilon probably made sure he had quite the surplus of regenerations when the Time Lords brought him back to fight in the Time War, the role could be taken by just about any other actor.
Here's a Super Six List of actors who could be considered:
1) Hugh Bonneville
2) Owen Teale
3) Nicole Walker
4) Julian Rhind-Tutt
5) Charles Dance
6) Helen Mirren