Black Adam is another, I guess..
Maybe a team up headed by Lex Luthor? It would be a way to work in some lesser known villains like Deathstroke, Bizzaro, Gorilla Grodd, Solomon Grundy, etc. They could even use that as a way to bring the Joker back in..
All that sounds like a bit much for the first movie though.
"I don't know how to put this...but I'm kind of a big deal"