This is how I would book it:
WrestleMania XXVIII: The Undertaker defeats John Cena, goes to 20-0 at WrestleMania. He's saluting the crowd in the middle of the ring and the lights go out completely. When they return, he's gone, and the urn is in the middle of the ring. He's never seen again.
My idea, on the Mania that loses he gets uo upset and frustrated the hwole arena seems to have gone haywire
Paul bearer jacks into the commentry and says"The spectreship of the Undertaker has now been calmed, he has no more unfinished business, its finally over, AHH YEAH!!"
Undetaker is now on the apron with an open casket appearing out of nowhere on the floor shouting "this can't be how it ends!" a bolt of lighing strikes him down he falls into the casket
his druids take him away and by the end of the night after the ending MV recpa we are seen somewhere outside the arena where the druids have buried Taker, they walk off, but one stays behind taking off his hood revealing to be Kane he stares at the grave and says "You really can now rest in peace, dear brother"
he walks off