The only way I can figure the Brock/Rock tease with WWE's assumed main event preferences this year is if Rollins cashes in during the main event, but that seems inexplicably stupid for Rollins to do and I don't know what the rules are then. Still, let's say:
Reigns / Lesnar beat each other up for 20-25 minutes and are both half-dead. Rollins comes down (with J&J even), cashes in and now it's a triple threat. They all go after Reigns, Rock's music hits and he clears out J&J, Rock Bottoms Rollins, eats an F5 from Brock, who turns around unprepared and gets Speared.
Reigns wins, still ostensibly a face, somewhat protected since he was saved from a lopsided beatdown and Rock didn't actually touch Brock, Rollins is still a heel, we don't have to worry about the cash-in anymore, and Brock thinks Rock cost him the championship.