Ryback/Taker would be awful. Ryback is hardly established enough to warrant a match with Taker, especially considering it would be Ryback's first Wrestlemania match ever.
And I'm just not sold on Ryback as a performer yet either. The guy still gets "Goldberg" chants in literally every arena he performs in. I'm not really sure a v2.0 type of guy is someone you want to end the legendary streak. Because if they go that route and Ryback never gets over afterwards, it would be a complete waste. And if Taker beats Ryback, then you can pretty much kiss Ryback's intimidation factor goodbye and it would be a waste of Ryback. If the guy had more of a track record, I wouldn't be as much against it, but he's just not ready for that match yet. I won't even bother make a list of everyone else who would be better-suited for the Taker match, but there's plenty better than him.
And I seriously doubt WM28 was Taker's last match. Though it wouldn't shock me if Taker skips WM29. I honestly wouldn't have a problem with that. If they have things planned for their bigger stars already and the only option for a Taker opponent would just be throwing any guy in there who's available, then I'd rather WWE not have the match just for the sake of it. Taker is a legend, but it would be overboard to say he's absolutely necessary to have at the event... as if WM can't be good without him. That's just not true.