What COULD she be whispering?
"I'm your daughter." (She's blondy from a few weeks back all grown up.)
"I'm your last wife." (She was there when he hits his recycle limit and inherited the book and screwdriver.)
"I'm Rose Tyler." (A stretch.)
"I'm your mother." (A bigger stretch.)
"I'm your father." (Eeeeewww)
"I'm your cousin, once removed on your mother's side. You remember your Aunt Hortense? Well, I'm her daughter by her second husband."
No, they aren't going to tell us. But there are clues.
A daughter wouldn't say "Hello, Cutie" to her father (Eeeeww again) not would she run her hand down dad's face like that. (OK, that answer is OFF the table. Agreed everyone?)
A (ex)wife/(ex)lover might do those things. She also has been time-traveling so perhaps she's now the owner of a/the Tardis ("the once and future Tardis?)
Or maybe she works for that same team of time-traveling hucksters that Capt Jack and Spike used to/still work for?