Killer Croc - Probably not a good choice, would need to be working with someone
Clock King - Could be a good villain and fairly open ended character meaning the movie could do what they want with him
Court of Owls - Not sure good as a second movie villain, but could be interesting later on. Would actually be amazing if they built up a series of movies with them pulling all the strings throughout.
Calendarman - Given the more noir style of the last movie, he could be an interesting one, but would probably feel too similar to the Riddler.
Some I could think of:
Killer Croc - Probably not a good choice, would need to be working with someone
Clock King - Could be a good villain and fairly open ended character meaning the movie could do what they want with him
Court of Owls - Not sure good as a second movie villain, but could be interesting later on. Would actually be amazing if they built up a series of movies with them pulling all the strings throughout.
Calendarman - Given the more noir style of the last movie, he could be an interesting one, but would probably feel too similar to the Riddler.
We’ve seen Killer Croc though, in Suicide Squad.
Court of owls seems like something build to.