Agree about not wanting to see Rock/Brock. It's not even the fact that it's two part timers, just that it's predictable and would pale in comparison with their 2002 feud. I'm also more eager to see what WWE would do to help promote Mania next year rather than just largely relying on The Rock to sell the show for them.
Brock will definitely be working with Punk in a rematch if he comes back for Survivor Series. Which I find disappointing, since if he and Punk were gonna work a rematch at the end of 2013, the hell in a cell match at the PPV later this month would have been most appropriate. Still, WWE believes that Survivor Series is still one of the big four PPVs of the year (which it isn't, as evidenced by WWE nearly scrapping it twice) so I can see why they'd have their rematch at Survivor Series to follow up their Summerslam bout.
If Brock doesn't return till the Rumble, then he's either costing Punk the match to set up their rematch at Wrestlemania or (far less plausibly) winning the WWE Title at the event so that Punk can win the Rumble and set up their rematch at Wrestlemania, which culminates in Punk defeating Lesnar and becoming champion at the same time. If the rumors are true that WWE want to turn Lesnar baby face after Mania next year, then that could be the logic of it... Heyman ranting at Brock for not only losing to Punk but losing the championship to him and the result being something involving Lesnar F-5ing Heyman or Heyman having Ryback attack and destroy Lesnar (setting up Lesnar/Ryback for later) or whatever. I really can't see Punk and Lesnar going at it at Mania with the belt on the line, though. I find it hard to believe they'd fight again at Mania at all, given the disappointing Summerslam buy rate, which the Punk/Lesnar feud shares part of the blame for.