As noted, WWE’s original plan for Sting at SummerSlam was for him to team up with Roman Reigns and Dean Ambrose in a six-man tag team match against the Wyatt Family (Bray Wyatt, Luke Harper and Erick Rowan).
However, with Rowan being out with an injury, WWE is looking for someone to replace Rowan in the match. It’s always possible that Sting will not wrestle at all, but just make an appearance.
In a minor update, Dave Meltzer said on last night’s episode of the Wrestling Observer Radio show that Sting is set to be brought into the storyline late in the game, likely as a big surprise.