Most assume Undertaker/Cena and Rock/Brock will happen next year, with Undertaker challenging Cena for his one last Wrestlemania encounter defending the streak, and Rock deciding to wrestle one last time but somehow I don't see them running two retirement angles next year, with two huge stars like Rock and Taker. Either Undertaker isn't hanging them up (even if he does fight Cena) or Rock will go out like Austin at 19, not mentioning that his WM match will be his last match (it'd be more of a non-kayfabe thing than something mentioned on-screen.) Or perhaps it leaves open the door for Rock to always return sometime in the future to wrestle again.
Brock/Rock will happen unless Rock is unable to wrestle. They've foreshadowed it, were originally supposed to start the angle off the night after Wrestlemania, Vince briefly mentioned it in a stock holders meeting and I can see the business side of it, pitting a Hollywood action star against the biggest draw of a combat sport like UFC. I guess I wouldn't mind seeing it.
If Undertaker doesn't retire next year, then Brock would be the best foe for Taker to run through. I want to see Cena/Taker over anything else, though.