True. And if Rock is available for Summerslam, maybe then he can continue his title chase. They can do Rock/Brock at WM, then Rock challenging WWE Champion heel Orton for the belt at Summerslam and completing his title journey on that PPV. Lots of possibilities but I still see Rock going over at the Rumble as 99.999% lock.
I'd much prefer something like this if Rock ends up facing Lesnar. The fact they're both part-timers puts me off from wanting to see them feuding for the WWE Title (even if they were full-time wrestler at one point). They can feud, they can be in the main event, get the most hype, whatever as long as the WWE Title isn't in there. Besides it's not like the match needs the title. A match like... say Punk/Ryback if they went with that would need the title a lot more than Rock/Lesnar. I'd much prefer Punk defend the title against Ryback and the WWE Title match being the third biggest match on the card (assuming Taker/Cena happens as well)... or hell, maybe even fourth biggest depending on if they go with Sheamus/Heel Orton for the WHC and how that's built up than them put the title on the biggest match of the show. And while that sounds very counterproductive towards the building of attention and credibility of the title, I don't think Rock or Lesnar winning it at WM and then vacating it the next night does it any favors past that Wrestlemania. As far as I'm concerned in the long run, if Rock isn't dropping the title to someone, it just brings the title back down to where it was and maybe even lower to vacate it.
If Rock ends up facing Cena, I'd rather Rock just take the title at RR and face Cena for it. It's gonna be obvious Cena is beating Rock anyway if they fight again, but with the title on the line it would add an extra dynamic to the feud/match.
I don't really care who wins at RR WWE Title match, so long as the right feuds happen based on the result (imo). To sum up, for Rock winning it's Rock/Cena (WWE Title), Taker/Lesnar, Punk/HHH, and Ryback/Show (WHC) (Though the most likely case it seems for this scenario is Taker facing Punk and Lesnar facing HHH). For Punk winning, it's Rock/Lesnar, Taker/Cena, Punk/Ryback (WWE Title) and Sheamus/Orton (WHC).
One other option is to have Rock win the title, face Ryback at Mania and put him over for the title, and I think that would be the best way to go as far as doing the best possible to keep the prestige Rock would bring to the title, while putting Ryback over as the main face in WWE (and to further cement that, Ryback beats Cena at Summerslam). Obviously though it wouldn't be the best choice to lead to the biggest buyrate they could get as there are a few better, but for long-term planning it may be the best way to go. The only issue I could see with it is if Ryback beating Rock turns people off, especially this early in his career. It worked for Lesnar because Rock leaving at that time was something relatively new at that point and the cause of a lot of resentment for him (on top of Lesnar being a beast), but now it wouldn't be the same. Rock would never get booed and Ryback is no Brock Lesnar anyway.