I think that Darkseid/new gods would be good
Darkseid or Black Racer
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
What do you think?