Yea, with Regeneration as a part of the show, there is no reason to do something so desperate as having another person take over the mantle of The Doctor. There's just too many ways they write in him gaining more Regenerations. Using the Trope of someone else taking over the Mantle of The Doctor, smacks of desperate moves such as Sliders Late Series Quinn or other such desperate moves.
If they were to go down anywhere near this road, it would be far better to go the Tremas/The Master route. Have some kind of accident that results in The Doctor taking over Capaldi's body and gaining new Regenerations. As far as The Elixir of Life giving him new Regenerations, that doesn't seem right, The Sisters needed to take it regularly, it wasn't a permanent solution taking it just once. I could however, see how it could be an "out" for not having used up a Regeneration becoming HurtDoctor
One Day I hope to be the Man my Cat thinks I am
Where are we going? And why are we in this Handbasket?