As far as being a weekly regular, I think those days are behind him. I'd like to still see him wrestle 3 times a year though. A quick token match at SummerSlam and Survivor Series and of course a biggy at Wrestlemania.
Look at Flair, Sting, Bret Hart, Hogan, Terry Funk, Lawler, Bob Armstrong, Jay Strongbow; even Freddie Blassie; they've all wrestled watchable matches past their prime. I really enjoyed Vince vs Bret at Wrestlemania and I loved Hogan vs String at Bound For Glory. They had the build; they told a great story and they had the pay-off.
That's all I need from Undertaker. He could have a great match with Del Rio and Daniel Bryan, trading blows and technical holds. He could have a 10 minute match with Barrett or Sheamus, telling a story and making them look good. All he needs to do it a punch flurry, walk the ropes, scoop slam, side slam, big boot, chokeslam and last ride. Done.
Doesn't matter if he's wrestling 5 star or even 2 star matches, all that matters is that he's wrestling. Once he's had matches with Bryan, Del Rio, Barrett, Sheamus, Ambrose, Rhodes and Ziggler I'd be happy to see him have a match with Cena and then retire for in-ring work.
I still want to see him on-screen though, maybe as a manager or authority figure of some sort.
As long as there are superstars on the roster he hasn't defeated then he can't retire.