He has only, what, nine TV appearances left? Those will either all be saved for the build towards WM, or they could squeeze in another PPV match before WM, since I doubt Brock/Undertaker needs to be built up with them appearing every week. I suspect it could/will be built up like Triple H/Undertaker at WM27, where they are never on the show but the announcers always keep bringing up both participants and talking about how great the match will be and who will win blah blah blah. So, assuming they wanna make use of all nine of those appearances that Brock has left, they could get a match out of him for Survivor Series (or wherever) and then build for Wrestlemania 29 as well.