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)