Originally Posted by Rorschach:
“The thing is she was sent to a parallel dimension following the battle of Canary Wharf in, what 2006?
So in theory the Doctor could encounter Rose any time between her birth and 2006. Just like Captain Jack said he had been to her estate and watched her now and again whilst waiting for the Doctor to turn up.
Sure it creates a problem with her not recognising the Doctor or the TARDIS when she later encounters them but I'm sure the script could get around that somehow.
Not that I want her back mind, but it wouldn't be impossible.”
2007.
If they are only talking about it with Billie now (if there is any truth in this) then it won't be anything to do with Series 4, all the scripts for that are long done.
The rumour is more likely to refer to the three specials to be filmed in 2008 and broadcast in 2009.
If (and I mean if) they did bring back Rose for these three specials then I would think we would also see David Tennant leaving at the end of them as I could only see Billie returning for DTs regeneration story.
Rose would either be killed off just before the regeneration or leave after refusing to travel with the 11th Doctor. If Martha or Donna are still around by that point they could provide the continuity to Series 5 or maybe TPTB want a clean slate and want to start Series 5 with a new Doctor and a new companion.