#1. Brock Lesnar vs. Seth Rollins at SummerSlam

Brock Lesnar, surprisingly, returned to WWE at Extreme Rules and cashed-in his Money in the Bank contract on Seth Rollins following Rollins and Becky Lynch's victory over Baron Corbin and Lacey Evans.
It came as quite a surprise because Extreme Rules isn't one of the big PPVs, which is where Lesnar has usually made his appearances, while the prospect of a title change happening at SummerSlam, which is just a month away, also made this quite a surprising decision.
With Lesnar now the Universal Champion once again, we could expect him to defend the title at SummerSlam - one of the big four PPVs of the WWE - and, I feel, that the only person against whom he could defend the title is Seth Rollins.
Rollins and Lesnar have a very personal rivalry and with so little time between Extreme Rules and SummerSlam (Lesnar is notorious for not making too many RAW appearances), we are certain to see a rematch happen at SummerSlam.