I don't think this really counts as return 'cuz he's only been gone for a season and a half, but the Master coming back would be nice.
Also a season finale kind of Dalek encounter would be cool (still doesn't count because it's only been the specials and a season and a half), but I heard we won't be seeing much of them because SM doesn't like the Daleks
Now to think of something that actually counts... The Tenth Doctor? There's three years of his life we didn't see (he became three years older between Waters of Mars and The End of Time), I'm sure they could work him into the next half of this season somehow...
