The Christmas Specials under Steven Moffatt were always set off to one side, with ideas and events based on fantasy they tended to be deliberately different to what you would see in a normal season of Doctor Who, and that is how they should be viewed and enjoyed to my mind.
As Ancient One said below there is no way a character like The Ghost could operate on earth in the Doctor Who universe without destroying the very need for Doctor Who!! He simply could not exist.
But the Christmas Specials are one-off's, you can do a character like The Ghost, You can have the Doctor meet Santa Claus, You can have the Doctor meet flying Sharks and living tree-men in a Winter wonderland...
The thought though that The Ghost even existed there in that episode opens up the question as to whether his existence means other superhumans existed on that world. Did, or does, earth have an 'Aquaman' perhaps? A man with links to The Sea Devils (remember them?!) would be a neat use of old continuity, is there perhaps an Osirian god out there waiting to fill the role of a Wonder Woman or Thor? Is there a Bowman inspired by Robin Hood perhaps?
Could there actually be an Earth out in the DR Universe that would be pretty much either DC/Marvel earth?