So will they get Piper too, or will it be pre-"Rose"?
There is certainly a theory that Nine had plenty of adventures before meeting Rose, although given how important Rose was to him as a new full-time companion, that strongly implies that if he did have pre-Rose adventures, it would be with a bunch of one-off characters in the companion role rather than one continuing companion. Which would mean lots of Nine-with-random stories.
But Billie Piper, Camille Coduri, John Barrowman and Bruno Langley are all doing BF already as their TV characters, so there's no reason there couldn't be stories with Nine set during season 1. Get Noel Clarke in and it would be the full set.
But then it's always been my belief that Nine was the one who first met Vastra and Jenny *, and the Paternoster Gang also have their own BF range, so this is the perfect chance to see that meeting and fill in that bit of backstory.
Combination of all three, throw in River because she gets bloody everywhere, plenty for twelve stories to cover.
When 12 appeared in "Deep Breath", Vastra said "Well then, here we go again." This was a Brigadier reference but it also indicates that Vastra has witnessed if not an actual regeneration then at least the aftermath of one before. The only regenerations of which we didn't see the aftermath on screen (and thus that Vastra could be present at) are Eight->War, and War->Nine. Since I also believe that the supposed "time lock" surrounding the Time War was really more about the War Doctor than it was the war itself (hence why no-one ever heard of the War Doctor and later regenerations could treat him as a secret), and it is reasonable to suspect that the War Doctor never came to Earth or else the war itself would have, then that only leaves a post-regen Ninth Doctor to be the one who met Vastra. His post-war trauma would mesh well with her assuming-she-is-being-attacked-by-subway-builders trauma, and they could heal each other while she meets Jenny and falls in love.
.