Update from the Observer
Right now, the two top matches for Mania are scheduled as Lesnar vs. Reigns and Cena vs. Undertaker. So unless Undertaker changes his mind, and given how hard they promoted him for the 25th Raw and his being backstage at TV in Birmingham on 1/9, it makes that seem unlikely, one would expect the angle for Cena vs. Undertaker to be shot in two weeks on the 25th Raw.