I agree on Constantine, but think it really only applies to the main team. Groups like Justice League Dark are really only named that for brand recognition. Him on a team dealing w. a magical crisis fits. He’s still an untrustworthy cutthroat bastard, but it fits.
I don’t love Deathstroke, but same as I feel about Deadpool as an Avenger, Slade should never be a part of the regular team, and man i’ve seen it suggested SO MANY TIMES over the years.