The person who hired Deathstroke may look to better their chances of Batman getting got by hiring a second assassin or perhaps they placed a bounty on Batman’s head and numerous assassin’s try to kill Caped Crusader. The only other assassin that comes to mind is Will Smith’s Floyd Lawton a.k.a. Deadshot. He was previously rumored to have a role in Affleck’s solo film and it would make sense that he’s the secondary assassin.
Wayne T. Carr was originally cast as Green Lantern for Zack Snyder's Justice League.
The promo art also shows Thor and Valkyrie.
Unfortunately, the sequel will probably never see the light of day.
Is a trailer right behind it?