You need to finish Shin Sekai Yori just to achieve closure - it's hellishly well-written and the ideas in it are so distressingly alien and yet humanly pragmatic that you'll weep for mankind. But you won't ever forget it either. Also it's creepy and the ED theme is a keeper.
In a similar vein, if you've not watched Gargantia on the Verdurous Planet you should do so. It's so NOT a giant mecha anime that you almost wish it was and you'll come for the scenery and stay for the genocide. I got a little annoyed at the (minor) fan service and the rather odd beach episode but only because I knew the show had more important things to do than pander to fans. Having watched it you'll be more alert in Darling in the FRANXX for the themes in the unfolding dystopian vision of that show - what goes around comes around I suspect.