Long time reader, First time poster.
It could play out either way with his last match either being this year and going into the Hall of Fame next year, or both next year, but WM30 will be built around the Undertaker in some form of fashion. I do believe he will go into the Hall of Fame the night before WM30 and his last opponent will be Cena.
Before I get into why it will be Cena, I do want to say this.. I completely disagree with letting any rookie have a match with Taker at WM, Let alone end the streak in hopes to "make an impact" for their career. The only person on the roster that is relatively new to "this era" that could be a possibility is Sheamus. Yes, he is a Triple h guy, and also just about every single interview he's done, the answer to his dream opponent, dream WM match, etc. has always been facing the Undertaker. Maybe if they nixed one of the HHH matches or if Punk hadn't have dropped his pipe bomb to get where he's at but Sheamus won't get the Undertakers last match.
Lets face it, Taker is finished after either this year or next. If we do get the opportunity to have him make it to WM30, his opponent will be Cena. I don't think Vince or WWE want to look back and make the same mistake they made with not having Taker vs. Rock or Taker vs. Austin at WM. Hell, I'm sure they're kicking themselves for not trying to set up Cena vs. Hogan earlier if the reports by Hogan are true. Whether you love or hate Cena, not having him face Undertaker at WM (not necessarily end the streak) just doesn't make sense, for many obvious reasons.
As far as if the streak should end, Although Punk has a list of accomplishments already that make him a future Hall of Famer.. to me it just doesn't feel right to have him be that guy to end the streak. How many ppvs in 2012 did he NOT main even being the WWE champion? That shows you what has been said numerous times. Cena for as long as he can go is "their guy". The word on the street about Punk only sticking around for a few more years, not to mention the injuries as of late that seem to get at him more and more. Punk has done a lot in his career, and so much in such a short time but not as much that Cena has done while in the spotlight. When it comes to adding it to someones list of accomplishments, it will go on Cenas resume. On top of all that, When Punk decides to cash out, I believe he will spend most of his time pursuing other interests and make appearances here and there where I feel Cena will stick around Hogan style.
If they were going to end it, Cena gets more heat than just about anyone night in and night out as a babyface so he doesn't need to be a heel in order to face Taker. Am I honestly the only one who sees him using his hustle, loyalty, respect routine to get a match at mania with taker because "he respects him and respects the streak, and win or lose it would be an honor to face him at mania" and take a crack at the streak. I see the ending being very similar if not identical to the way HBK ended Flairs career. Cena in the corner with that look in his eyes going for the 3rd or 4th AA. He doesn't necessarily want to be the one to do it but hey..Never give up, right? If the streak ends, it end like that. I honestly think at this point that Vince and WWE keep the streak alive not only for the reasons of showing gratitude to Taker for his years, the gimmick that brings in money and sells tickets, but for fear of 80 some thousand people causing a riot or more realistically the negative feedback and possible loss of fans.