I wasn't expecting too much from an Apocalypse mode except more dangerous natural disasters.
I was expecting at least more souped up/threatening climate change, possibly some different apocalyptic scenarios that could (or could not ) be prevented in some fashion, and some form of increasing roving bands of barbarian hordes (whether dressed up as some fantastical like 'zombies' or not).
Granted some of this could still happen, even in conjunction with other game modes (i.e. a 'plague' game mode that creates an apocalyptic plague when turned on with apocalypse mode), but I'm not super optimistic.