River says PLEASE!! I know you aren't interested in pulling in a Doctor, but any companion would do. Any at all. OMG- Amy or Rory would have built in plottage. Captain Jack would be all kinds of amazing. And could you see Ace?? Or Donna? Or Sarah Jane (omg, back story!) But seriously, I'd take anybody. Someone she can be snarky with, who understands the Doctor, but will NEVER understand him the way she does. Because she's River. End of story. She knows his name! She's his wife! She's just the best thing that ever showed up in his long life, and no one can compete with that.(OMG, she won't shut up!!)