I don't see CM Punk heading back into the mid-card scene. It's a waste of his talent and WWE knows that. If WWE isn't planning on keeping Punk as a top card guy, I doubt they'd have rushed to sign him back the way they did around this time last year.
From top to bottom, Punk has had the strongest & freshest WWE Championship reign in years. Despite not being pushed as hard as Cena, Punk has continued to garner main event level interst & responses from the fans week in and week out for more than a year now. If anything, I see Punk's run as champion as being something of a test and it's a test that he's been passing. Otherwise, I don't think Vince would have kept the championship on him for nearly 7.5 months.
It's all but written in the stars at this point that, at some point in the relatively near future, it's going to be Cena vs. Punk II. We might all be wrong but the general feeling that Cena will win the WWE Championship MITB match at the upcoming ppv. Punk currently has two monster wins over John Cena, once at last year's MITB and again at SummerSlam. When you take that into account and couple it with the fact that Cena has been out of the WWE Championship picture for close to a year, it's easy to think that Cena is due to balance out the scales and it's possible that he might. At the same time, however, that's not the virtual iron clad guarantee that I think it would've been had not a couple of things happened:
1. Punk's strong WWE Championship reign - I pretty much already discussed this earlier in the post but one thing I wanted to touch on a bit more was Punk not being pushed as hard as John Cena. Cena is still THE face of WWE, that can't be argued, but Punk has continuously been involved in solid to strong feuds for the title, often delivering show stealing matches at house shows, television & ppvs. His fan response is right up there with Cena and has been that way since last year.
2. John Cena losing to The Rock at WrestleMania XXVIII - The fact that John Cena lost the overall biggest match of his career, to me, is a sign that Cena is no longer the nigh invincible juggernaut, aka SuperCena, that he's been portrayed as. The loss to The Rock has seen Cena become a bit more "human" than we've seen in a long while. He lost a match to Tensai on Raw, lost it clean when you take into account it was a no DQ match. He also needed the help of 5 other men to defeat Big Show in the cage match at the NWO ppv. If Cena had gone over The Rock and had continued just being the same ultra dominant Cena we've seen in recent years, I don't think there'd be any argument that Cena would be the one to take the title from Punk. Cena's not being booked quite that same way, however.
I think WWE has realized that they do have to really start building other main event guys to one day take the reigns from Cena. On top of that, WWE has built Cena to such a level of star power that he can lose and not really suffer any loss of stock because of it. Hell, if anything, Cena's stock seems to be higher among most fans than it's been. His loss to The Rock, his brutal match against Lesnar, his feud with Show have humanized him in my eyes. He's overall been more interesting I think. If Cena does win MITB, he's someone that can cash in the contract and lose without it really damaging him.
As for when Punk's title run will end, personally, I think it'll last somewhere around a year, maybe more. If it goes to WM 29, I'll be very surprised but I certainly won't be disappointed.