Caves should also have random rats as well, smaller rat sprites rather than the gigantic kind that are fightable enemies.
Same for dungeons.
For the Future, definitely some randomly sparking machinery in factories where electricity is still on, but in places like Proto Dome where there is nothing, I'd suggest something more like the occasional panel breaking down, and perhaps random rats too. Not very many on the random rats though...there'd not be much food anywhere.
Dragonflies are good for the Prehistoric, along with randomly falling leaves. Perhaps tiny little primates can also occasionally wander around places, something like minature lemurs or ferrets.
Bubbles and pufts of white are also good. Perhaps the occasional drop of sweat in some fashion for warm areas such as the Prehistoric, if that is manageable.