No. 2 - The Rock
You might be asking yourself, "What business does The Rock have at SummerSlam? He's not making a full-time WWE return anytime soon. He has no business making an appearance." If that's what you're thinking, I hear you. But respectfully, shhhh.
The Rock doesn't need a reason to show up places, he just does so whenever he pleases. The man was at WrestleMania a few years ago just so he could set a giant metal version of his name on fire and announce the attendance record. He even made an appearance at a Boston live event a few months back. A live event!
On any given night, Rock's music could hit, he could cut a promo, maybe make fun of one of the heels, and go on his merry way and no one would question a thing. He oozes charisma and entertainment. If WWE wants to add a little extra oomph, albeit by doing something that has no long term consequences, they could call up Rocky and throw him into the ring for 15 minutes or so. Why not? Stranger things have happened.