Earl Spencer (Diana's brother/Will's uncle) and his wife named their daughter, born in July of this year Charlotte, so I think that could potentially put that one off the table. Maybe not, but I suspect they wouldn't want to repeat a family name so soon, even if it is the "other side" of Will's family.
If it is a girl, it won't be Diana, not as a first name, (a) because it's not a traditional royal name and (b) because she'd forever be compared to Diana. William wouldn't want that for his (potential) daughter.
Queen Victoria left one hell of a legacy, and history teaches us she was a formidable monarch, and one wonders if the name is a lot to live up to. If they have a girl first, perhaps this might sway them from the name.
For a girl, my money is on Margaret, after the Queen's beloved sister, or indeed Elizabeth after the Queen. That also honours Catherine and Carole (both have it as their middle name).
For a boy, I think maybe George, or perhaps Alfred.