I read a similar thread on the old WWE Universe site. On that thread I read a post from a user who got the idea that once he got to the point of 20-0, 'Taker should get the privilege of a Ric Flair treatment. Going in the Hall of Fame while still being an 'active competitor' on the roster. So basicly inducting him with this year's Wrestlemania and compete at the same event.
As for going out. That post read that he should be able to take one random star in the bussiness during the period between New Year and the Royal Rumble (preferably one that needs a boost) and put him up in a program with Undertaker, where he would be the manager of the selected star.
He said that if it was a star who is already on the roster, then Undertaker should persuade him to the 'dark side' with promo's and vignettes. So he could learn him everything of being a man of darkness.
If it were a development star, it should be someone who was already portraying more a darker character that just needed a boost to get over with the audience. (Much like you have now The Ascension, Bray Wyatt or even Dean Ambrose with a small character tweak)
That way WWE management can decide to use a pupil challenges master storyline where the officials could decide whether to make him win or lose against the star of 'Takers choosing, dependable on the audience's reception. He also stated that if he were to lose then, 'Taker should lose with style. Going out in a way to remember.
This was his example:
The gimmick of the match itself doesn't matter as long as it is the final match of the card. After a hard fought battle Undertaker finally gets pinned following ...'s finisher. The winner can't believe that he just laid out Undertaker and the crowd stands baffled but cheers for this. As the Undertaker lays in the middle off the ring, the superstar go around the ring to celebrate his hard fought victory with the crowd. As suddenly the lights turn off... the infamous gong hits and 'Takers blue lights turn on.
The only thing the light highlight is the ring and we see that Undertaker has disappeared with only his hat laying in the ring. The superstar hits the ring, puts up the hat as a lightning bolt hits the stage. On the titantron we are shown a pre-recorded video (for if this case were to ever happen) where Undertaker thanks the superstar for a hard fought battle and says 'you have earned the right to be called the "new phenom of the wwe"'. As the video ends, the superstar lays down the hat as Undertaker's music hits and the superstar raises his hands as the show goes off the air.