4) A Major Character Death
The death of Black Canary in Arrow did not go down well, though that was mostly because the show killed off its best female character instead of the two who have become unbearable (Felicity and Thea). If this four night crossover is indeed going to be some sort of epic event unlike anything we’ve seen on TV before, then it needs to have ramification, and the death of a beloved character would go a long way in ensuring that so long as it’s not just done for the sake of it.
Captain Cold’s demise in the season one finale of Legends of Tomorrow showed the best way to handle this type of thing, and providing one of these shows can find an equally epic and emotional way to say goodbye to a hero, then it should happen here, especially if it has an impact on whichever show they hail from moving forward. It’s hard to say who this could be, but a Legend seems most likely.