I think most of fandom wants the JLA to face off against Darkseid, but I'd like to know which of their villains you'd like the JLA to face off against AFTER that.
The Injustice Gang
The Weapons Master
There ARE more, so if you wanna "write in" your choice, go ahead. Personally, I'd like to see Dr. Light minus the Infinity Crisis update they gave him to appear. Make him dangerous like he was in his first few appearances.
The Shaggy Man and Queen Bee (original) are high up on my list, too.
A fully Lovecraftian Starro would be a LOT of fun.