He is only Batman's villain

Deathstroke and Redhood

Poison Ivy

Black beetle

Deathstroke is already in the arrowverse

Godspeed again

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.