Ryback vs Taker would be a huge rub for Ryback but yeah, I agree with Pumpkinhead that Ryback isn't worthy of facing Taker for the streak at Wrestlemania.
Taker vs Big Show at Wrestlemania 29? Nah, doesn't really seem like a worthy match of drawing a lot of interest.
In my opinion, it should be Taker vs Lesnar or Taker vs Punk next year in the Metlife stadium. It depends if Taker is healthy enough to compete at Wrestlemania 29 and if not, have him miss next year's Wrestlemania 29 or at least have an involvement at Wrestlemania 29 that doesn't necessarily have to be competing in a match. That way, it gives Taker a break for a bit and it also saves Taker vs Cena at Wrestlemania 30 (or at least I hope