Mentioned more than once, so imo too important to not be *someone* who will be revealed at a later date. But who?

Personally I wouldn't be surprised if it was NPH. We know that whatever is being planned was set in motion long ago, and centred around Donna (and Rose through her). Rose was also making and selling toys, which is too much of a coincidence, and this is how she's brought back into events to play a much larger role in part 3.