Does anyone ever think that perhaps we read too much into how things are done? I haven't searched the forum, but I'm sure there must be speculation about her being Donna, River Song or Romana.
So they haven't announced Karen Gillan's character name yet. Perhaps it's simply down to the new production team. Maybe RTD liked fans to know the companion's name from when the actress is announced, but Moffat doesn't want to do it that way.
Or maybe he's having fun because he knows it will start all the Donna Noble, River Song, Romana blah blah speculation. Perhaps it will be one of those characters, but I really think it will be a completely new companion. Personally I don't think Moffat wants to drag anything form the RTD era into his.