In that case, having Punk retain the WWE title until at least January is the right call. Ziggler could then chase Punk's unified championship for a while, with loopholey GM talk justifying his MitB contract as still perfectly valid, and when he eventually wins it'll be revealed that it only truly affects the old WHC, resulting in the two titles being split up again with Punk holding on to the spinner.
Or, delay the unification ceremony until the RAW after Hell in the Cell, and have Ziggler crash the party then, but with the same WHC stipulation as above. In either scenario, Punk and Ziggler come out looking strong and the only real casualty is Sheamus, who'd benefit from a refresher angle anyway. Months of fighting off ADR has done nothing for him or the WHC. Let him chase for a while, let him be hungry again.
If they must insert Cena somewhere, he could assist Ziggler in some way so that Punk doesn't necessarily have to lose the supposedly-unified title to Ziggler cleanly.