Regarding Roman Reigns possibly challenging Brock Lesnar for the WWE World Heavyweight Title after Night of Champions, Reigns will likely be kept out of the title picture until he wins it at WrestleMania 31.
Reigns is still the “chosen one” but the idea behind his push right now is for him to rise to the top and having him lose to Lesnar at this point would be counterproductive.
If Reigns won the title now instead of at WrestleMania, they would miss out on the big WrestleMania moment and it would not maximize the impact of his title win.