For those asking, yes, the plan is for The Undertaker vs. Brock Lesnar at SummerSlam. WWE has already produced advertisements for the cable and satellite systems that include The Undertaker.
Also, as noted a few days ago, Sting is scheduled to wrestle at SummerSlam. He will be involved in a match with Bray Wyatt. The way things were set up at Battleground, I would guess that the match will be Sting and Roman Reigns vs. Bray Wyatt and Luke Harper. Harper and Wyatt reunited at Battleground.
WWE plans to load up SummerSlam because they want to boost their WWE Network subscriber numbers and because they want to make it a huge weekend. There will be a 2-hour live NXT special taking place at the Barclays Center on the night before SummerSlam. There have been plans for the past two years or so to make SummerSlam into something that is almost as big as WrestleMania.