1) She is the Rani
This old chestnut always comes up... the Rani was a Time Lady played by Kate O'Mara back in the 80s. Many people want to see the Rani character given another chance, thinking she was a victim of being in the wrong time and the wrong place; the least watched era of the show.
Likelihood: Slim
Pros: The character does have lot of potential.
Cons: Outside of fandom no-one really cares about her
2) She is Susan
Susan was the Doctor's "grand-daughter" and companion back in 1963. Well, she called him grandfather, but there's a question mark over their literal relationship.
With the 50th Anniversary coming up next year, Doctor Who fans are feeling even more nostalgic than usual and many would welcome such a nod to the show's past. It would also be intriguing seeing the youngest Doctor ever playing granddad.
Likelihood: Possible.
Pros: The last few series have seen a number of references to the show's history, the library card with William Hartnell's picture, for example.
There's also a passing resemblance between Jenna-Louise Coleman and Carol Ann Ford, who played Susan.
It would be a "game-changer" and could lead to the return of the Time Lords in Susan's wake.
Cons: Steven Moffat is on record as saying he's no fan of the Time Lords and it would be strange (although not impossible) to bring back Susan without them.
To date, no classic character has yet been "regenerated" back into the show like this. The nearest example is the return of Sarah Jane Smith, played by the much-missed Lis Sladen.
3) She is related to Rose/Donna/Amy/River
We've just had two series where the main companion played a central part in determining events, so it has become something of a habit to expect the companion to have some familial relationship with other characters - could this trick be repeated again?
Likelihood: Not on your nelly.
Pros: Fans seem to want it...
Cons: I'll stick my neck on the line - first of all, there's no chance that one of the Tennant companions will be alluded to so strongly, it would be a pointless exercise for the show.
Secondly, for all Steven Moffat's bullishness, he does respond to feedback from fans. There was a lot of criticism of Amy's coldness and one-dimensionality during Series 5, answered with her return as a much warmer, richly emotional character.
And there is still noisy criticism from some quarters over Amy's supposed dominance - the universe seems to revolve around her and her extended family... I don't think Moffat would choose to continue down this path with a new companion.
4) She's The Doctor's Daughter
Specifically, this means Jenny, the Doctor's Daughter from the episode of the same name. There's a hardcore bunch of fans who would love to see this character brought back - she was kick ass, gorgeous and the Doctor has a complicated relationship with her.
Likelihood: I very much doubt it.
Pros: Intriguingly, the future for Jenny was left open, is she on her way back?
Cons: Part of Jenny's popularity came from the comely charms of Georgia Moffet, who is now married in real-life to Doctor 10, David Tennant. Okay, we could explain that Jenny's blonde hair and blue eyes turned brown due to a regeneration, but do you think the writers would be bothered to go to all that trouble?
5) She's a minor character we've met before
Have you heard of Sally Sparrow? She grew up a bit more, cut her hair short and is now working in Hollywood under the pseudonym Carey Mulligan. The leading character from Blink became a fan favourite, with many calling for her return. Similarly, Lorna Bucket (seen in A Good Man Goes To War) has prompted an excess of fan speculation and wishful thinking from people who envisage her as a future companion.
Another recent character with companion potential was Lily, the daughter in The Doctor, the Widow and the Wardrobe. She looks a little like Jenna-Louise, don't you think?
Likelihood: Possible.
Pros: You can see the temptation of reintroducing a character we've already met, there's the chance to play around with time and re-appraise episode(s) they previously appeared in.
And the Doctor has met a lot of interesting young female characters over the years (especially in stories scripted by Steven Moffat), from Nancy in The Doctor Dances, through to Lily.
Cons: Before the announcement about Jenna-Louise Coleman was made, many claiming to be in the know said we would see the return of Reinette (Sophia Myles) and that didn't happen did it?
Also, why cast a new actress when the original ones are around.
6) She's someone completely different
Now, that sounds much more like it.