It has to happen at some point, the franchise of the WWE against the future franchise of the WWE. The big question is, will it happen as soon as SummerSlam? Or will WWE save it for something like WrestleMania 32? There's a few reasons to believe WWE might start building it for the August show. Key is that Cena's feud with Kevin Owens will probably have concluded after July. As for Reigns, he needs a big match, but the WWE title will be preoccupied with Lesnar and Rollins. Thus it seems like a good time to do the Cena Vs Reigns match. It has headline potential and would be an entertaining match. It's the big win that Roman Reigns needs, and he'd perhaps earn the cheers against someone like Cena.