User blog comment:JMOfficial/Future Crossover Villains/@comment-24152851-20170420034249

An expanded Legion of Doom would be awesome. Though maybe they'll undergo a rename to Crime Syndicate of America.

Agree with Anti-Monitor. Crisis on Infinite Earths may be just what we need to merge Supergirl's universe with the main Arrowverse, so they can freely crossover Kara with the other shows.

Lot's of us were talking about Darkseid as another potential crossover villain. He'd definitely be a big enough threat that would force all of the Arrowverse heroes to unite.

I'll also add Nekron and the Black Lantern corps to the list, for if they get around to adding the Green Lanterns to the Arrowverse. Even if they don't, they could change it so Nekron just resurrects dead Arrowverse characters without the "black lantern" part. This would be a good chance to bring John Constantine back as well.