After imagining eleven different incarnations of Susan, reflecting or responding to each of the Doctors, who else could be next but the Master?
It should be noted that the Master is much better at regenerating consistently (and coming back from the dead) than the Doctor, so he might not change as wildly as the man he’s the shadowy reflection of. But he still has room to change pretty wildly...
The Master was a contemporary of the Doctor, so the first and second Doctors could meet him before the Third does quite readily. We already have prototypes for the Master in the Monk and the War Chief, but it would be easy to imagine a more straight-up scheming Master (plain black outfit, goatee in place already) up against the previous Doctors. Maybe taking the War Chief’s place, but not the Monk’s.
The Third Master we know, of course. A good guide for an ongoing villain with a variety of tricks up his sleeve, as well as possible interpretations of the role.
The Fourth could be Geoffrey Beevers, in and out of zombie makeup, replaced late on by Anthony Ainley as the part-human cracked Master.
He went on to clash with the Sixth and Seventh Doctors, but a flamboyant Six and a treacherous Seven would be easy enough to see. Or perhaps, in contrast to the Seventh Doctor’s trickery, the Seventh Master is plain-speaking and upfront in his dealings, young and affable where the Doctor is fatherly and mysterious.
The Eighth Master starts off by drezzing for the occasion, but the DWM comics show a grimmer, more serious version, fighting the Doctor to prove his point and pretending he doesn’t like it.
The Ninth Master, perhaps Sir Derek Jacobi? Angry, frightened, running for his life and prepared to sacrifice anyone for his own safety.
The Tenth Master we know. Indeed, we know two very different versions of him, an example of the character’s versatility.
Looks like we won’t get an Eleventh Master, but I can see a playful young chap who just occasionally lets the smile slip...
There’s plenty of room with the Master for various schemes, the subtle puppeteer or the shrieking supervillain, fixating on the Doctor or ignoring him, and each could work well. Do we want an amiable Master, an over-the-top Master, a frightening Master or a fun Master, a mirror image of the Doctor or a polar opposite? Ideally, of course, we should see a hint of every aspect...
And what would a multi-Master team-up be like?