I wished Taker/Lesnar happened this year. But in the case of Rock not wrestling and Taker not hanging them up yet, then Taker/Lesnar was always the obvious alternative. Taker even said in an interview in March that he wants to face Lesnar at a Wrestlemania one day. Undertaker/Cena is so much better for the 30th Wrestlemania though, but I guess Taker may indeed be trying to go to 25-0... Disappointing.
If a newer guy got a shot at the streak though, I could see Bray Wyatt being a dark opponent for Undertaker. Undertaker up against a twisted and demented cult leader sounds entertaining. The only flaw there is that while the build sounds good, the match itself might not be. I know match quality doesn't mean much for the casuals, but Taker's had the best (or second best) match of every Wrestlemania since Wrestlemania 23/24. Giving the best or one of the best performances of the night is clearly one of Taker's motivations for Wrestlemania at this point (JR talked about this in an interview), and I don't see being able to get one out of him against Wyatt. From what I know about Bray Wyatt's ring work, he's not typically someone you'd expect match of the year from. Same goes for a Taker/Ambrose match, likely.