Well, Rose will most likely not be coming back since she's got her own Doctor now (uh...huh) and she is linked to Jack and Jack is linked to Martha (as in how they met) and Donna's lost her memory. They probably all feel that they don't need the Doctor anymore (poor man). Jack is probably the most likely to turn up again (in two parter episodes. He always seems to do it). As for Sarah Jane, who knows! River Song is definetly going to be a future companion but as to when, I don't know!
Maybe if the Doctor found Romana...somehow. Because the Time Lords resurrected the Master to fight in the Time War, who's to say that Romana got trapped in a very cold fridge, survived nuclear hellfire and turned up in a UNIT base after the fridge (being blown off a planet from the nuclear bombings) crashed into Earth only aging slightly (well, look at this pic of her
http://i5.photobucket.com/albums/y173/Sabalom_Glitz/LallaWard.jpg ) without any broken bones and meets the Doctor, slaps him for blowing up Gallifrey, slaps Martha for being in her way to slap the Doctor and slaps Sarah Jane for getting at the Doctor first in the 1970's!!!! Cor, try saying that in one breath!