If someone had to end the streak, it would have to be John Cena. Yeah, you could give it to a young up and comer, but there's a problem with that. Remember Brock Lesnar? He was the definition of beast. He went over guys like Rock, Undertaker, Rob Van Dam, Hulk Hogan, etc. He became the younngest WWE Champion ever (and although Randy Orton is the youngest World Champion ever, Lesnar is still the youngest WWE Champion ever). He was booked as a dominant fighter, and then he...left. Just got up and left. He came, got his World Title, fucked his 5 Divas, and then left.
So that's why it should be someone who isn't a newcomer, or isn't an old timer. If they made Undertaker lose to Shawn Michaels last year, then it wouldn't do any good, because he's leaving next year. Same if he lost to Kane or Triple H. They're only going to be around for a couple more years. So to make it believable, and prepare for their future, Cena needs to win. Cena has shown his loyalty to the WWE. He has enough star-power to pursue a movie career, much like The Rock. But he has stayed with McMahon. He will be around for another 10 years, at the least, barring injury. As for Cena being booed out of the building, well as long as Cena doesn't get too cocky about it, that shouldn't be a problem outside of the hardcore smarks. Cena doesn't look like the type to do drugs either, so he won't fall to the drug tests, either. Punk would also be ideal, but he has been reported to have attitude problems, and while he might be drug-free, he looks like the type to get mad and leave. So I say Cena is the way to go.