Partial Source: The Wrestling Observer Newsletter
The current plan for Brock Lesnar is to have him win the WWE World Title at some point (likely SummerSlam or Night of Champions) and then possibly go on to defend it against Roman Reigns at WrestleMania 31.
As previously reported, Lesnar will be working SummerSlam and Night of Champions, so those are two possible spots for him to win the title. WWE is planning to "stack" their Night of Champions pay-per-view as they want to get their current Network subscribers to renew their subscriptions when the six-month block ends in the fall.
It remains to be seen what Lesnar's schedule will be like after Night of Champions. Obviously if Lesnar holds the WWE Title from SummerSlam to WrestleMania 31, he'd presumably have to work more dates than his part-time schedule usually calls for.