A feud between Taker & Lesnar heading into WM 29 has an epic feel to it and there's a decent amount of history WWE can draw upon in order to give the idea that The Streak is in jeopardy. Or, I should say, if Taker leaves WM 28 with The Streak intact.
Of all the opponents Taker has had in his WWE career, Lesnar is really the only one that I know of that ultimately got the best of Taker. I have a feeling things would've been evened up, eventually, had Lesnar stayed in WWE but he didn't. Lesnar's clean win over Taker in the HIAC match was a great, competitive match that came down to the wire. It looked like it could go either way but Lesnar came out with the win, a clean win. I think he also beat Taker in a street fight match on either Raw or SD!, but had help from Paul Heyman in scoring the win. It wouldn't take any real effort at all to build Lesnar as the greatest threat in the history of The Streak.
As for a Lesnar vs. Rock match, honestly, it doesn't really interest me that much. For one thing, it'll just feel like two guys who're hot in the mainstream media just showing up for a WM match. Lesnar has been gone for nearly a decade as has The Rock and I'm sure Rock isn't going to be staying around to work even a part time WWE schedule. Because of that, it'll just have the feel of WWE going out and paying a shitload of money for a couple of mercenaries who're just there. I'm sure it'd generate some big buys for the ppv but, for me, there's no real emotional connection to be made in this match. To me, Taker vs. Lesnar feels like a spectacle with a deep story behind it. Rock vs. Lesnar just feels like a spectacle, all style and no substance.