If they wanted to go with a finish of Cena defeating The Undertaker and ending The Streak, I think if Cena modified his STF into a chokehold and choked Taker out after a grueling 20-30 minute match would be believable. They can continue the story of Undertaker being the WrestleMania war-torn veteran and Cena bringing a vicious force upon the physical vulnerability of the aged & beaten Deadman. They also could of course go this route and Undertaker can survive yet again in victory and ultimately choose to walk into the sunset putting a close to his legendary career himself in a respectable bow out. Whatever occurs in the grand finale of Undertaker's 20+ year career, it needs to be EPIC, FITTING, and GREATLY MEMORABLE. I would never forgive the WWE if the ending of the Streak was just a throwaway accomplishment on someone's resume. This moment, if it is to happen, must have major ramifications on person/character that does it and has a venerable impact on the business.
My mental image of Cena/Undertaker is, Cena is, after sustaining a Chokeslam and Last Ride fairly early on in the match, Cena turns the corner by completely destroying Undertaker. I even thought of the bit of Taker passing out (though only momentarily) in the modified STF, only to just barely hold on at the last second. Cena AA's him several times through the course of the match... Taker locks in Hell's Gate and Cena shows off his strength by picking Taker up and powerbombing him several times in succession. Hits him with several more AA's but Taker keeps just barely kicking out. One more AA attempt, but Taker slips out of it and Cena turns around into a jumping Tombstone ala WM26 and is pinned.
Taker stands and then kneels victorious in the middle of the ring as the show goes off the air.
At the end of the next night's Raw, Cena compels Taker to confront him, and manages to get the upper hand in a wild brawl with the aged Deadman. AA's him through the announce table, takes a chair and pummels him into pudding. STF choke. More chairshots. Whips him through the barricade, hits him in the head with a ringbell. Haggard Taker unwittingly leads the brawl up the Raw stage, where he puts up one last offensive in the fight, sending Cena down with multiple strikes. A spent Taker hovers over the fallen Cena, and as he has so many times in the past, motions (though more wearily than usual) for the Tombstone with the throat-cutting gesture. He bends down a little to try to begin picking Cena up, but Cena fights back and simply pops up to his feet with Undertaker wrapped around his shoulders. AA off the stage, out of camera range, Cena stands at the top of the stage, looking down, end of Raw.
Either Cena would be a heel going into the whole Streak storyline with Undertaker, or the storyline would turn him heel. In any event, he would forever be able to claim that he retired The Undertaker from WWE, even though Taker actually kept the Streak intact against him.
Next January, Heel Cena has ruled over WWE as champion for seemingly ages, a younger apparent "underdog" star of the future is lined up to challenge the superheel Cena. The face has a moment of doubt about his chances in ever dethroning Cena. Suddenly, the lights go out and Undertaker finally reappears and gives the most emotional pep talk ever, lol, saying that he lived and breathed WWE and finally found himself consumed by it through Cena's iniquity and that if Babyface X doesn't take WWE back from this sinister overlord, Taker's metaphorical death was in vain, etceteras, etceteras. Undertaker should finally display pathos in discussing his career, his singular standing and role in the entire industry, as he says with heartfelt anguish that the time of the Deadman has finally come to an end at the hands of John Cena, and that it is time for the next generation represented by Babyface X to take their rightful place through the trial by fire. Undertaker says that while his body has been fundamentally laid to rest in the world of WWE, his soul still burns brightly, etceteras, and that with that in mind, he is rendering that unto eternity that which is eternity's, and is offering himself to be inducted within WWE's hallowed Hall of Fame (with HoF tickets going on sale the Saturday after this week's Raw in January 2015, lol), etceteras, etceteras.
Bottom line, I don't think the Streak should ever be broken. But you can craft a storyline that nevertheless carries the full measure and weight of that Streak as Undertaker's career comes to an end.