I think that Darkseid/new gods would be good
He is only Batman's villain
Deathstroke and Redhood
Deathstroke is already in the arrowverse
Zod for supergirl, Harley Quinn or the court of owls for Batwoman and Darkseid for the legends.
Can't think of anything for flash
Those sound awesome they should be
Barbatos for a crossover.
What do you think?