Brock Lesnar’s next match will in fact happen at SummerSlam, as has been the expectation. Dave Meltzer confirmed that Lesnar will wrestle at SummerSlam on August 19, in what may be his final match before his current contract and stint with WWE expire.
Though his opponent is still unknown, the most likely person is of course Roman Reigns, though WWE could surprise everyone and have it be Bobby Lashley or someone else. Braun Strowman getting a match seems less likely, since he has the Money in the Bank briefcase and it would make more sense for him to make a surprise cash-in rather than have an official match with Lesnar.