No. 4 - Paige
For a while, it seemed unlikely that Paige would ever set foot in a WWE ring again. Between the video leak, her relationship with the WWE-bashing Alberto El Patron, and most importantly the issues with her neck, her WWE career looked over. But according to Paige herself, she's going to be returning soon.
It's unclear what her medical status is or if she'll be cleared to return by the time SummerSlam rolls around.
But even if she's not, assuming she expects to be cleared in the near future, she could still appear in a non-physical role. She could bolster whichever Women's division she jumps into (Raw probably needs her the most) and she would create a huge moment in Brooklyn if her music hits.
Maybe she shows up and knocks Alexa Bliss' lights out after the Women's title match. Maybe she does the same to Naomi.
There are plenty of possibilities if Paige shows up at SummerSlam, all of which would create a memorable scene.