Essentially, he's Moffat's version of the Woman in White from The End of Time. The Woman, of course was intended by RTD to be the Doctor's mother but her identity wasn't stated on screen so that she could be whoever the fans wanted her to be.
I thought she was Susan. Put photos of them side by side, you'll see why.
As to Baker, a later (but not final) regeneration, going back to an old favourite face. And yes, they don't need to do any more than that, leave it there.