#1 Shinsuke Nakamura

The most logical and perfect choice would be to reveal Shinsuke Nakamura as Reigns' attacker. The current Intercontinental Champion, Nakamura hasn't been seen on the show for some time now and he didn't feature on the SummerSlam card.
It seems like the WWE creative team doesn't really know what to do with the King of Strong Style, so it would be perfect for him to be revealed as the Big Dog's attacker.
It would put the Intercontinental Championship back on the radar and it would also mean that for the first time since Reigns' return from illness, he would be involved in a little challenge.
The feud between Nakamura and Roman is a dream one for the WWE Universe and if Nakamura is to succeed as a heel, he has to face a top face of the company. It is almost likely that the attacker would be someone from the mid-card, I hope it comes out to be Nakamura.