Does The Undertaker still have "it". If "it" is the power to draw money, crowds and heat, the skills to perform moves and holds safely, great ring command and the determination / dedication required to put of some of pro-wrestling's all time greatest matches then the answer is: yes.
Unfortunately, Mark Calaway is 47 and has put his body through hell for our entertainment and clearly is unable to keep doing so, so if "it" is the ability to still "go" in the ring, sadly the answer is no. While I respect the hell out of what he's done in wrestling (he's 2nd in my all time top 3 favourite wrestlers) and I don't mind him working one match a year on the grandest stage of all it's time for him to retire. He has no-one left to beat, nothing left to prove and nothing left to do in wrestling. As for beating Cena; John Cena is a different kind of Superstar to The Undertaker he's The Ultimate Warrior to 'Taker's Randy Savage, he's Hulk Hogan to HBK. A win over Cena would change nothing and mean nothing because The Undertaker should always beat John Cena.
I really hope that he and WWE can find a way to have him involved in WWE programming and maybe Wrestlemania but if The Deadman has worked his last match all I can do is say "thanks for the memories, the terrifying, awesome, 'GOOD GOD ALMIGHTY, GOOD GOD ALIMIGHTY THEY'VE KILLED HIM' memories."