Not sure why they bothered keeping it a secret, but oh well. At least we definitely know who the 11th girl is going to be. And OAG did seem to imply in one of the articles that the Switch version at least will add additional girls akin to DOAXVV's method of adding girls into the game.
Thinking about the pattern of new girls added to DOAXVV: we can hope Tina is the next one.
New - Luna
New - Tamaki
Classic - Leifang
New - Fiona
New - Nagisa
Classic - ???
New - ???
New - ???
Classic - ???
Well, if the in-game code from October and November is of any indication, we're probably going to get CRI, MIL, and RAC next, which would point to Christie, Mila, and Rachel. Still, I'm waiting until February to verify this from my source (and I chose February since that's closest date I can ask before the next girl launches, which I estimate is March 14 based on past release dates). Unless DOA6 reveals "Tina" is short-hand for Christina, anyway, like in the film adaptation.
Oh, and before I forget, here are the special salutations for the New Year: