I don't want a WW foe in this. But... gun to my head? Circe. In the background as set up for her appearance in WW's solo. She can be a shady and powerful figure Lex is wooing. He knows she's trying to use him, and she knows he's trying to use her.
And call me nutz, but since Trevor has never been much of a consistent or interesting character and cuz for a loooooooonnnnnng time Mera was out of the picture, when I was younger I always thought that Diana and Arthur made a logical pair. Crown princess of the Amazons that worship the female Greek deities and the King of Atlantis that worships Poseidon. Flashpoint sort of makes that pairing toxic now, but that's sort of the way I see it. And now that both Trevor and Mera are back it's kind of moot.
As for treaties and the like, I sort of always thought that DC was always rife with hidden societies. So I think it would be cool to see them band together in some way, even if it were in some kind of mutual defense pact. Gorilla City, Paradise Isle, Atlantis, Nanda Parbat, Skartaris... That's a pretty powerful group and it ensnares a ton of the JLA into such a story.
But for this film? No. I think Lex and at most two more of the usual Superman foes is all it needs. Say Parasite and Metallo.