Constantine’s appearance could have something to do with Sara specifically, although when Constantine is involved, anything paranormal or involving the afterlife is fair game.
While it would be cool to have Constantine a part of the crossover, at least it means things can’t get any weirder for the heroes participating in that event.
While the exact episode Constantine may appear in is a mystery, the fact that Legends of Tomrorow is specifically mentioned would appear to indicate it won’t happen during the mega-crossover.
It’s been a while, so Arrow fans can be forgiven if they’ve forgotten it was John Constantine who was responsible for resurrecting the soul of Sara Lance.