User blog comment:OneOfThoseCrazyGirls/5 Villains Who Must Appear in the Arrowverse/@comment-27700324-20180418161546

Nekron wouldn't work. You just need Green Lantern before you make Blackest Night and that doesn't seem to be happening.

Godspeed seems too early since he was only recently introduced. So was Reign, but at least her comic book story arc completed so they have material to use for the show.

The others are good possiblities.