It'd be a waste of the stipulation. Traditionally, the MITB briefcase IS for elevating wrestlers to the main event spot. Guys like Edge, Punk, RVD & Miz were well known up & comers but they didn't ever the main event scene in WWE until winning the briefcase. Kane is the odd man out in terms of winners, so it's not out of the realm of possibility to use the case to push someone into the main event scene that's been there before.
However, Kane isn't The Rock. The Rock is on a level of star power, overall talent & fan interest that Kane could never dream of reaching. The Rock is such an icon in WWE that he can just show up, demand a title shot, he'd get it and nobody would miss a blink. Even a good deal of internet fans, who generally piss and moan about everything, would really complain about WWE going that route BECAUSE he's The Rock. He's one of the biggest stars that's ever been or ever will be in pro wrestling.
On top of all that, according to The Rock himself via Twitter, it's unlikely he'll be making very many, if any, appearances in WWE for the rest of the year. He's already got three other film projects on his plate right now, so I doubt we'll be seeing him until it's close to Royal Rumble time.