Regardless of whether the rumors are true, Brock Lesnar would undoubtedly be The Undertaker's greatest WrestleMania opponent and challenge ever.
The match would be about more than just Undertaker's streak or who the better wrestler (or fighter) is. The match would be about Undertaker's love for the wrestling business vs. Brock Lesnar's love for Brock Lesnar.
Both men have achieved great success in WWE. However, success is probably the only similarity that the two wrestlers have in common. The two men couldn't have taken more different paths. While The Undertaker made his WWE debut at the 1990 Survivor Series and hasn't left the company since, Brock Lesnar debuted the night after WrestleMania X8 and left just two years later at WrestleMania XX.
While The Undertaker has dedicated his life to the wrestling business, Brock Lesnar seemingly has been unable to commit to one "sport." While The Undertaker has stuck with WWE through thick and thin, Brock has gone from WWE to football to UFC and now temporarily back to WWE.
If John Cena made The Rock look bad for leaving WWE for seven years, can you imagine how bad The Undertaker could make Brock Lesnar look for leaving WWE for eight years or for quitting when he had the WWE in the palm of his hands?
Ultimately, if Brock Lesnar were to face The Undertaker at WrestleMania XXIX, Undertaker would not only be fighting to keep his streak alive, but he would also be fighting for the professional wrestling business and every single part of it—from the wrestlers to the fans, past, present and future.
There's one more reason why Brock Lesnar would be The Undertaker's greatest WrestleMania opponent and challenge ever: Brock Lesnar is an "a**-kicker," not a "superstar." Brock demolished John Cena at Extreme Rules and later broke Triple H's arm. In other words, if Undertaker were to face Brock Lesnar at WrestleMania XXIX, he would be facing his most devastating opponent ever.
Ultimately, if Brock Lesnar were to face The Undertaker at WrestleMania XXIX, Undertaker would be fighting for his streak, his love for the business and to prove that he is still the phenom of the WWE.