Nobody stays dead
It's now got to the stage where you would be very hard put to find any character you could be reasonably sure would be pushing up daisies on a permanent basis. Hell, I would have considered Cir-El a safe bet and then she got a cameo in one of Jeph Loeb's guest star fests, which means that she's still alive somewhere.
I can't honestly think of any character too iconically dead or generally disliked that there isn't a real possibility they won't pop up again. I'm expecting Alex DeWitt to climb out of that refrigerator any time now.