I think that Darkseid/new gods would be good
Hush, Trigon, Solomon Grundy, Killer Croc, Black Mask, or Bane.
Joker is the main villain DC and he should be included in Arrow-verse
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
What do you think?