“Oh, hello.” Those two words, from the person Ncuti Gatwa‘s Doctor regenerated into in the Doctor Who Season 2 finale, kick off a slew of questions we now have. Because that person? Billie Piper, very recognizable to fans as Rose Tyler, the companion of the Ninth (Christopher Eccleston) and Tenth (David Tennant) Doctors who also has a major connection to the TARDIS itself.

Most notably, the end credits, which usually read, “Introducing [Actor] as The Doctor,” were different. It only said, “Introducing Billie Piper.” She has yet to be credited as The Doctor — and not just there. The statements from Piper and showrunner Russell T Davies are carefully worded as well.

“Billie once changed the whole of television, back in 2005, and now she’s done it again! It’s an honour and a hoot to welcome her back to the TARDIS, but quite how and why and who is a story yet to be told,” teased Davies. “After 62 years, the Doctor’s adventures are only just beginning!”

Added Piper, “It’s no secret how much I love this show, and I have always said I would love to return to the Whoniverse as I have some of my best memories there, so to be given the opportunity to step back on that TARDIS one more time was just something I couldn’t refuse, but who, how, why and when, you’ll just have to wait and see.”

Furthermore, Piper’s “to step back on that TARDIS one more time” suggests that she may only be back for a limited time.

So what exactly is going on here? Below, we take a look at some possible theories.

Billie Piper is the Sixteenth Doctor

It could simply be as straightforward as any other regeneration, and Piper is the Sixteenth Doctor but along similar lines to David Tennant returning as the Fourteenth. She’ll be around temporarily, for one or more specials, before regenerating again, and someone else will be the Seventeenth Doctor for Season 3. After all, she is still wearing the same clothes as Ncuti Gatwa was.

Billie Piper is a Doctor

Piper could be playing a Doctor but not one that’s going to count sequentially. Maybe we have a War Doctor (John Hurt) or Fugitive Doctor (Jo Martin) situation going on. Maybe there’s a little Rose in her — after all, Rose once looked into the TARDIS and the TARDIS looked back into her, when she became Bad Wolf to save the Ninth Doctor in the 2005 Season 1 finale “The Parting of the Ways.” Maybe Gatwa’s Doctor triggered something when he used his regeneration energy to punch into the time vortex to bring back Poppy. (Read a recap here.)

Lending credence to this (or what follows) is Piper’s post on Instagram following the episode. “A rose is a rose is a rose!!!” she wrote in the caption, alongside tagging the handles for Doctor Who and Davies and including the emojis for a rose and a wolf! See it below.

Billie Piper is playing Rose again

Again, no one has actually called her the Doctor just yet. Between that and Piper’s post, plus some timey-wimey shenanigans — that description did come up again in this episode, with both Ncuti Gatwa and Jodie Whittaker‘s Doctors saying it — and some similarities between “Reality War” and the two-parter in which Rose was last seen, “The Stolen Earth” and “Journey’s End,” in 2008’s Season 4, maybe somehow Rose was brought back from the parallel Earth on which she was trapped/lived.

In “The Stolen Earth,” Earth and other planets were one second out of sync. In “Reality War,” time shifted by one degree, and the Doctor had to change that (using some regeneration energy, ultimately triggering the regeneration) to get Poppy back. In “Journey’s End,” Rose said that she had built a dimension cannon to make her way back from her parallel Earth to the Doctor’s, only the whole of reality started to collapse. In “Wish World” and “Reality War,” the Rani’s (Archie Panjabi and Anita Dobson) goal was to collapse reality to bring back Omega from the underverse. Is this a bit of a stretch? Maybe.

But what if the Wish World was collapsing followed by what the Doctor did to bring Poppy back were just enough to bring down the walls between the two Earths as well and somehow this is now Rose standing in the TARDIS? As for where the Sixteenth Doctor could be, it could be on her Earth, it could be something else timey-wimey. Whatever the case, this could be a brief time of Rose being back before she returns home and we get the next Doctor.

Billie Piper is playing Rose again — and will reunite with the Fourteenth Doctor

The above can be true but there could be another twist coming, and yes, we know this is probably the least likely to actually happen. But what if this is Rose, and what if this is Doctor Who‘s way of pairing Billie Piper and David Tennant onscreen again? (We’d love to see it.) After all, Tennant’s Fourteenth Doctor — following the bigeneration that introduced Ncuti Gatwa’s Fifteenth Doctor — is still out there.

What if Rose has come back and is going to reunite with the Doctor and they’ll finally get their happily ever after? Yes, Rose ended up with a version of the Doctor — the Metacrisis one, part human and part Time Lord, that grew out of the Tenth’s hand in the aforementioned Season 4 two-parter — but the Time Lord never got the chance to tell her he loves her or to be with her. And “Reality War” saw Jodie Whittaker’s Doctor wondering if she should tell Yaz she loves her only for Gatwa’s to reveal she doesn’t. Maybe this is going to be the way of fixing that for the Tenth Doctor, who let his Metacrisis counterpart finish what he hadn’t when he and Rose first said goodbye to each other when she became trapped on the parallel Earth in the 2006 Season 2 finale “Doomsday.”

Billie Piper is playing Bad Wolf

We can’t forget Bad Wolf, especially not with Piper’s use of a wolf emoji in her Instagram post. Bad Wolf created herself in 2005’s Season 1, spreading the message across time and space to lead Rose back to Satellite Five in the year 200,100, to save the Ninth Doctor and wipe out the Daleks. When the Doctor sent Rose away, the only way for her to get the TARDIS to return to him was to look into the TARDIS and the Time Vortex. The Doctor now, in “Reality War,” punched his regeneration energy into the Time Vortex. Maybe he triggered something. Could that be why also during the regeneration sequence, there’s a cut away to the TARDIS console?

But who do you think Billie Piper is playing? Let us know your theories in the comments section below.