First and foremost, it's pretty difficult to end such a gimmick as undefeated in Wrestlemania. It would be a tough decision, but it would leave a bad taste in everyone's mouth if such gimmick is suddenly finished. If it does get finished, it won't be by DiBiase Jr. I'll explain why.
From what I've heard and seen around (which isn't entirely credible, but at least makes a lot of sense), Taker's last match is meant to be in a Wrestlemania, and against a wrestler that could really offer a chance to give a good match. Right now, the Taker/HBK match follows all those parameters; hence, a good position for a surprise would be Taker announcing retirement in the midst of 'Mania 25. If he decides to take ONE more year, his shot for best retirement match ever will be crippled. There's very few stars that would give Taker the match he deserves to end his career. Michaels is one of them, but a rematch for next year seems flawed. From the remaining people, you'd either need Kane getting the mother of all promotions to give Taker one last match (because of all the baggage and stuff), make it a sort of Gauntlet match with as many of his 'Mania opponents as he can (imagine: a 10-17 vs. 1 match as a "return of all the phantoms of the past"), or a match between Taker and his logical successor. Boogeyman isn't there anymore, Kane is a jobber that will probably be dismissed before Taker retires, and there isn't a "dark" wrestler gimmick around, unless all of a sudden a young new star decides to continue the gimmick.
A Cena/Taker or HHH/Taker match would be like an insult. Worse if any of them actually makes him lose his streak; Taker much like Michaels can run any match by itself and make it spectacular, but the real skill of both of them would be pretty much well shown. Aside from that, few wrestlers now and then would give a really good match (the closest exception would be a Jeff/Taker match, given that Jeff keeps his moody gimmick)
Second, once they retire, they'll be inducted after five years. Unless there's a very specific exception (such as Ric Flair's induction), they'll be inducted after a while.
And third, DiBiase Jr. has to prove himself he's a wrestler beyond a gimmick to truly deserve to be the opponent in Taker's retirement match. Right now, aside from his spot at Legacy, he hasn't shone by his own light; rather, you could say he's doing PR work for Orton. In a sense, he's like Morrison; a really good wrestler that hasn't gotten the chance to shine alone. The main difference is that Morrison is almost past his due to strike it alone (one of the reasons why I desire the Colons to win; so that the Miz/Morrison pair disbands, Miz fades into oblivion while Morrison becomes a heel main-eventer) Give it two or three years, and let DiBiase Jr. polish his skills a bit; maybe a bit more of mic time, and he'll be main-eventer in no time at all. Before that, he's not material to retire Taker. Michaels is right now the best material to retire Taker, hands down, because he's an established legend on his own.
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