, your mentioning Tom Baker as the villain Kurra (sp?) in The Golden Voyage of Sinbad
reminded me of something.
I read something once, most likely apocryphal, that Baker would not have minded continuing with the production if if he could have played a villain, particularly the Master. Consider how Kurra looked once he dropped the talisman segments into the enchanted fountain and it rejuvinated him. Remove the turban and slick back his hair, and he'd look like a successor to Roger Degado. Again, I'll point out this more likely just fannish rumor run wild, but I see the perfect opportunity where Baker's Master could have been introduced.
In this scenario, there would not have been a "Logopolis", at least not the way it unfolded. Instead, the regeneration story would have been the previous one, "The Keeper of Traken". Remember, the "burnt bacon" Master as played by Beevers, wanted to absorb the rest of the Doctor's regenerations for himself using the Source Manipulator. As depicted in the actual story, his efforts failed due to Adric's sabotage. But he gained enough power to take over Nyssa's father, Tremas and continued in that form until the cancellation of the original series.
But...I see how it could have played out very differently (at least "in universe" if not snactioned by the real life production). What if the Master were partially successful? I can imagine he starts his "leeching", but only manages the Doctor's current form as played by Tom Baker, but not all his potential regenerations. The Master takes on the guise of a van dyke bearded Baker and the Doctor himself is forced into a regeneration, introducing the next performer to play him (we'll assume it's still Peter Davison). Given the limitations of the BBC effects, I picture something akin to a simple "blurring" with a wash of light as Beever's scarred form grabs Baker, maybe "clawing" at his face. The optical makes the figures grow indistinct as both collapse. Adric and Nyssa approach the Doctor's form and remove a length of scarf that has fallen across his face to reveal his new form. The other body, still in the burnt robes rises with a menacing chuckle that only one actor can mange. He whips back the cowl to reveal Tom Baker with a mustache and beard rather like the reinvigurated Kurra from Golden Voyage...
Cue the "sting" for the closing credits and the wait for the next season (series).
Again, I highly suspect Baker's comments about playing the Master are just that, suspect, but there was a perfect point in the series where Baker could have switched roles. And remembering him as Kurra, I think he could have made the role his own.