The big crossover event this season, “Invasion!,” merged most of the casts of Supergirl, The Flash, Arrow and Legends of Tomorrow for a four night event, so, technically, any characters from those shows could appear on Legends at the end of the season.
Wow, these possibilities are already getting kind of complicated, aren’t they?
Unless, of course, Legends uses an Earth-2 or Earth-3 version of one of those characters, which we actually haven’t seen before and could account for a big surprise when he or she shows up.