Cool that the Sisterhood of Karn were mentioned. Too bad Brain of Morbius was such HORRIBLE episode. Grade F monster flick.
But the Sisterhood of Karn are one of those elements that has become so central to Doctor Who lore and yet never used or heard of again, possibly for good reason. Their connection to the Timelords is huge, and the possibilities are endless, maybe too much so.
BTW: for anyone who worries about the 12-regeneration limit, there are a million ways around that, some of which are very obvious canon from long ago. The Master himself is testament to that, as he supposedly was on his last regeneration back during the 4th Doctor. We know the Timelords can offer a whole new regeneration cycle, as they promised The Master for his aid, which he still managed to not give even with that incentive! The Sisterhood of Karn seem capable of doing crazy things with Timelord regeneration. The Master was able to body snatch his way into another regeneration, with all his abilities intact (though we'd assume he only has one heart). But The Timelords giving The Doctor a new regeneration cycle for saving their butts? They'd be stupid not to.
In fact, I half expect that Moffat will do that soon. If Capaldi is the final regeneration, that brings up another problem: Final Regenerations are weak. The master looked like a rotting corpse during his, though one other Timelord was referred to being in his final regeneration and looked just fine, so I'm not sure what that's about. I fully expect that the 50th anniversary will end the Time War, the Doctor(s) being the savior. If so, then the Timelords owe him that much. Hell, they go on and on about Rasilon, but The Doctor has saved and elevated their kind more than the old geezer.
The fact is, the whole "12 regenerations" was made canon during the 4th Doctor (I don't believe it was during Pertwee), when the creators had NO IDEA that the show was going to go on for that long. They probably made it 12 regenerations as a way of making the number so high that they figured we'd never have to worry. It was never a number to hang over our heads, so I have no problem with the creators making some easy way out of it, in order to stick with the original intentions of the show.