Of the known / leaked characters;
Joker - Injustice Joker is dead, Home Joker went home.
Canary - Taken by Dr. Fate to another dimension to which he claims he will not return.
Arrow - Injustice Arrow is dead, Home Joker went home, Canary Dimension Arrow should still be in Canary Dimension.
Darkseid - Punked by Superman twice, should have technically withdrawn from this reality with the New Gods.
Dr. Fate - Sucked into the void along with Trigon and Mxyzpltk
Swamp Thing - Trapped in hell after Flash fails to save him
Scarecrow - Dead (Mobile says he was resurrected by The Rot for some reason).
But as I said, given the super flimsy excuse Tom gave for MMH, we should all accept every one of them are about to get some poorly thought out all passes.