On Raw Monday night, Reigns did appear in the opening segment, along with fellow SmackDown stars Daniel Bryan and Kofi Kingston. Vince McMahon was present inside the ring with the three blue-branders, giving him an opportunity to announce a change to how the brand split works.
Per McMahon, a "wild card" rule is being instituted, in which three SmackDown stars can appear on Raw, and vice versa. As of this time, it is not clear how often this rule will be invoked, if it'll be weekly, monthly, etc.
In theory, the idea does keep things on either show from becoming too stale, as the notion of random appearances from off-brand performers could add a touch of unpredictability to the shows. With SmackDown moving to FOX in October, WWE would likely want to be able to rotate different stars into that all-important telecast as they see fit.