David Tennant has teased that there are “many other people” who filmed for the Doctor Who 60th anniversary special(s) on a closed set.

The actor, who played the Tenth Doctor and is returning, alongside Catherine Tate as Donna Noble, for celebratory episodes next year, was seen filming for Doctor Who on location with Tate, Bernard Cribbins, and special guest star, Neil Patrick Harris. However, at a recent convention in Boston, Tennant refused to give away any plot points, except to say that the photos so far “aren’t even close to the full story”.

And if “many other people” filmed on a closed set, that means we should have some surprise cameos…

But who?

Obviously, as it’s an anniversary story, you’d expect more than one Doctor to show up. Matt Smith, who played the Eleventh Doctor regularly between 2010 and 2013 (then again in Deep Breath the following year), is an obvious choice, seeing as he’s a much-loved incarnation who fronted the 50th anniversary special, The Day of the Doctor with Tennant; he’s not returned since either, except for that aforementioned cameo in Peter Capaldi’s debut story.

Similarly, we’re always looking for more screen-time with the Eighth Doctor, Paul McGann, whose mini-episode, The Night of the Doctor, was one highlight of the 50th anniversary year.

Neil Patrick Harris also said he had “interact[ed] with [new Doctor, Ncuti Gatwa] a bit and he’s glorious”, so could that mean we’ll see Gatwa’s incarnation in the 60th anniversary storyline? He’s reportedly due to start filming Doctor Who proper in November, but we could see him cropping up before then anyway.

Right now, there’s a lot of speculation and very few, if any, answers. River Song? Jenny? Susan? K9? Martha Jones? A Zarbi? Missy? Jamie McCrimmon? Your guess is as good as ours. Fun times, indeed…