The only thing I want to see less than Rock/Lesnar is Rock/Undertaker. At least with Brock, Rock actually had a damned good match with him ten years ago, while Rock and Undertaker never matched up well with one another, never had a decent, much less memorable feud or program with one another and seem to be a couple of characters that are just better off not entering one another's orbits. These problems are only magnified and exacerbated by their respective statuses today, with Undertaker and his sanctified Streak on the one hand and Rock and his movie star celebrity on the other.
Triple H just tried to fucking murder The Undertaker at consecutive Wrestlemanias. And Triple H could do that because of who he is and what character he boasts coming into the ring. Rock isn't going to revert to his Corporate Rock asshole heel self from a decade and a half ago and start bashing Taker with a chair like he did with Mick Foley once upon a time. He's a completely different Rock, nobody would want to see that, nobody wants to see him turned heel (to what point?), none of it benefits WWE aside from whatever buys they may collect because it's a Big Match. But not every possible Big Match is the correct Big Match.
That's why Lesnar vs. Undertaker makes so much more sense. Triple H was a killer in his matches with Taker at WM27 and WM28. Now that killer was just taken down by an eviler, hungrier, more vicious, more ruthless and sadistic monstrous killer in Lesnar. The Streak at this point requires someone who's willing to attempt to murder Taker to break it. Even John Cena at WM30 could bring that to the table after all the years of battling the Umagas, Sheamuses, Lesnars, et. al., fighting in Last Man Standing matches, Elimination Chamber matches, etceteras. Cena bridges the gap between the Triple H model and The Rock model but he's at least believable in that role, and best of all he's in a different place in his career and we don't know what he'd look like, exactly, as either a heel or at worst a heavily-booed "tweener" (which is what he'd be in any Streak storyline by de facto).
Rock can't really play that role anymore. He can still bring a certain edge to his character and he can even be booed by a segment of Cena diehards throughout the audience but he can't--won't be allowed to--return to that territory.
I think it's unquestionable that Rock either pursuing the championship or eventually wearing it as champion will certainly elevate the championship. This is actually a role which he can fill quite capably and in so doing he can actually elevate something, namely the WWE Championship. As was said by SharpshooterSmith, simply having Rock/Cena II for the WWE Championship ensures that the WWE Championship match will headline Wrestlemania as it logically should.
Don't necessarily agree with the last bit. Theoretically speaking, if Cena were to turn heel, a long reign as a heel overlord of WWE would do the WWE Championship, the John Cena character and the WWE at large a world of good. Of course, even as a babyface he can maintain the status quo... which is not what most of us want but it wouldn't exactly sink WWE, it seems.
Anyway, WM29 is probably going to be, to one extent or another, used to set up WM30... Kind of like how WM27 was used to set up WM28. So while it'll probably be more strictly "predictable" than, say, WM28 (though, really, did anyone think Sheamus, Undertaker or Punk weren't going to win?), much of it will be used as the jumpstart for WM30's big angles (Cena being set up to challenge the Streak, Punk's elevation as a true top tier player after WM29's program, doubtless the snowballing of certain rising stars toward WM30, etceteras).
Rock and Taker have never even been booked to have a meaningful feud between each other. The only high profile 1-1 match they had was Rock v Taker at King of the Ring 99 and that was just a filler match in between Owen dying on the previous PPV and Austin winning the belt the day after King of the Ring.
I wonder why that didn't feel special or memorable?
The selling point of a Taker Rock Wrestlemania match would be you'd have both guys trying to cement their legacy as the greatest Wrestlemania performer of all time. As big as 20-0 is you can make a legitimate argument that Rock's wrestlemania history is better. The guy has had dream feuds and victories over Austin, Hogan, and Cena. The faces of the era. The streak is a big list of average wrestlers and B+ stars.