Interesting question. They already killed [BLACKOUT]General Zod[/BLACKOUT] in MoS so there is probably gonna be casualties, villains or good guys. Probably the latter.
A league member will not die, that´s for sure. It would be one thing if they decided to go with Abin Sur to pull in Hal Jordan that way or if Barry dies so that Wally can be introduced if ,say, they wanted different elements and charachters for the big and small screen.
A hero tho doesn´t have to be superhero, hence a leauge member. I guess you meant Superhero? I´ll throw out some names of the likes of which could die. Jimmy Olsen, Trevor, Dr Silas, Dr Ivo, Vandal Savage (you know what I mean, come on now), Black Adam, and I save the best for last, Gorillas from Gorilla city. They could easily die. They´re gorillas + Grodd is really evil.
I don´t think they´ll kill bigger charachters though,[BLACKOUT] Zod[/BLACKOUT] was different and I was surprised that he was killed but it tells us DCCU can be dark.