If Undertaker was going to retire after Wrestlemania 30 maybe they could announce it now. Do a year-long farewell tour. That would get fans to attend the shows since he would probably be there, get one last chance to see him. It would give them the ratings icreae they want especially with Punk, Rock, and others gone indefinitely. Create competition (like how Punk won chance to face Taker) for people to want to face him at PPVs. It probably wouldn't work or WWE would ruin it, but that's an idea to keep Taker around for a full year this time since it would be his last and give him the respect he deserves. Just throwing it out there..
Maybe he has a few more WrestleManias left, WrestleMania 30 would be a good place for it, but so far they've really downplayed the idea of the 30th WrestleMania. I remember when WrestleMania 20 was announced they were making a huge deal of it, with the "Where it all begins again" tag line. We're still obviously still a year away so we'll see. I don't see how his opponent will be anyone other than John Cena though, unless they do the rematch with Punk and save Cena for 31