Undertaker and Sting are arguably my 2 favorite wrestlers of all time. My top 3 is a combo of Sting, Taker, and Steve Austin. So while Taker/Sting is a dream match, it's also 10 years too late. Sting is like 55, haven't watched TNA recently but Sting the past few years has wrestled in a t-shirt over his attire, so he's not in good shape. I just don't think it would be a good match. I think Taker will be fighting Lesnar or possibly Cena, depending what the title situation is.
Regarding CM Punk, I can only hope this leads to him winning The Royal Rumble match. I can kind of see the Authority doing something where they make Punk enter at #1 and he goes the distance and wins. This would lead to a match with Orton or Cena or maybe even a triple threat match. Either way, Punk has to win. This is WrestleMania 30, if you end another Mania with Cena standing tall people will go crazy. WrestleMania 20 ended with Benoit and Guerrero standing in the middle of the ring as champions, so they need to do something different here. Punk needs his signature Mania moment and this is it.