I think that Darkseid/new gods would be good
Flash: Red Death, Mirror Master, Shade, Cobalt Blue
Batwoman: Red Hood, Bane, Black Mask
Legends of Tommorow: Trigon, Despero
Supergirl: Starro, Ultraman & the Crime Syndicate
Crossover event: Darkseid (Adaptation of JL Origins which officially forms the Justice League in the Arrowverse) or Blackest Night with Nekron (Hopefully to confirm a Green Lantern show)
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?