WWE is advertising both The Undertaker and Chris Jericho for appearances after WrestleMania 29.
Jericho is being advertised for the 4/22 Raw taping in London, however it's likely that he will not be appearing on any other Raw shows between April 11th and the 18th as he will be touring with Fozzy. He has a day off on the 22nd and Fozzy will already be in The UK, so Jericho will appear on the show.
Jericho recently stated in an MLW.com interview that his current run with WWE is expected to wrap up shortly after WrestleMania.
As for The Undertaker, he is being advertised by London's O2 Arena for the 4/23 Smackdown taping. Beyond that, it's unknown whether that will be a one-off appearance or if 'Taker will appear regularly for WWE after WrestleMania.