'AGENTS OF SHIELD'
"THE ONLY LIGHT IN THE DARKNESS"
In the SHIELD bunker hidden away in the Great White North, Agent Eric Koenig subjected each member of Phil Coulson's team to the lie detector chair designed by Nick Fury. Koenig started off with a series of questions to establish a baseline.
Among those questions: "You wash up on a deserted island, alone. Sitting on the sand is a box. What is in that box?"
There were only two answers that leapt out at me. One was from Fitz; he wanted Simmons to be in that box.
As for the other, Simmons answered without hesitation: "The TARDIS."
There was no splainin as to what the TARDIS was, nor the source of its existence. This isn't too suprising as Joss Whedon oversees this show's production and he is a writer who trusts in his audience to understand his pop cultural references.
As far as the Toobworld Dynamic is concerned then, she was speaking of the TARDIS as though it actually existed.
However, 'Agents Of SHIELD' takes place in an alternate TV dimension, that of "Comic Book Toobworld". This world is made up of 'Agents Of Shield', 'Lois & Clark', 'Birds Of Prey', the Marvel-based movies about 'Captain America', 'Iron Man', 'Thor', and perhaps 'The X-Men', Tobey McGuire's 'Spiderman', and the incarnation of 'The Fantastic Four' which got two movies under their belts.
It's easy to believe that the Doctor would have a doppelganger in this dimension. After all, he's appeared in quite a few comic books, sometimes even interacting with the regular characters from the company's other comic books.
I'd be curious as to which incarnation of the ever-regenerating Doctor currently resides in Comic Book Toobworld. Does his life (lives?) mirror that of the Doctor from the main Toobworld? It could be that he's still a few regenerations behind. Then again, aging has not been kind to most of the still extant actors who played the Doctor. So maybe the Time Lord looks like somebody beyond the appearance of Peter Capaldi's version.
I feel confident in stating that the Doctor will never be showing up in 'Agents Of SHIELD'. But if he did, purely hypothetical of course, I'd like to see Frobisher (who accompanied the Sixth Doctor as his companion in other media.) With CGI, that could very well be accomplished and the producers could get a cool actor with a recognizable voice to bring life to that sapient penguin!
And it could be that Frobisher is the companion to some other incarnation of the Doctor.......