WWE is planning on bringing in part time and Attitude Era stars for RAW going forward for the next six months before WrestleMania 29. The purpose of the move is to make sure that the relatively name-value talent roster doesn't get over-exposed and to prevent problems such as the major ratings dip last week when John Cena wasn't on the show.
The company has three particular names that have been tossed around; the first is Undertaker who could make an appearance in the Royal Rumble match. He is already planned to have a WrestleMania match but the Rumble would get him back earlier.
The Rock is also an obvious choice, as he already has a Royal Rumble match against the WWE Champion. There has been internal discussion about bringing him back within the next few weeks to boost the ratings, though it isn't clear if that can happen due to his movie schedule.
Steve Austin is also a possibility, though he would not wrestle for obvious reasons. He could be involved in a TV angle, especially with CM Punk dropping his name in recent promos. The company has been having Punk drop Austin's name to promote WWE '13, but they may decide to use that to bring him for a storyline of some kind.