Tim, Cassie, and Conner are stuck in the phantom zone so it's possible they could come back. But Conner was sent there by Superman to save him because he was dying.
http://injustice.wikia.com/wiki/Categoryeceased
-Scarecrow (Jonathan Crane)
Jimmy Olsen
Lois Lane Kent
Kid Flash (Bart Allen)
Beast Boy (Garfield Logan)
The Joker
Nightwing (Richard "Dick" Grayson)
Kalibak.
-Martian Manhunter (J'onn J'onzz)
Captain Atom
-Green Arrow (Oliver "Ollie" Queen)
Year Two
Kyle Rayner
Ch'p
Despero
James Gordon
-Arkillo (Sinestro Corp Member)
-John Stewart
-Guy Gardner (2nd Green Lantern of Earth)
-Mogo the Living Planet
-Ganthet (Guardian of the Universe)
-Black Canary (Dinah Lance)
Year Three
-Jason Blood (Etrigan's human form)
-Harvey Bullock
-Ragman (Rory Regan)
-Deadman (Boston Brand)
-Phantom Stranger
-Klarion the Witch Boy
-Detective Chimp
-Huntress (Helena Bertinelli)
-Swamp Thing
Year Four
-Renee Montoya
-Hercules (Greek Demigod).
-Artemis (Amazonian Warrior)
Year 5
Year 4 Annual)-Kilowog
-Parasite
-Weather Wizard .
-Heat Wave (Mick Rory)
-Trickster (Axel Walker).
-Bizarro Superman
-Alfred Pennyworth