Thanks for your answer. Fog would be nice. For example like in Carpenter's 'The Fog'. Bloodmoon night starts and there rise a very dense fog in the area around you. Zombies come out of this fog and attack. Dead Zombies vanish in this fog (so the game just deletes them this way), and a 'Fog of War' for unknown terrain could maybe even tweak game performance slightly?
There's already a MOD with something similar: Punishing Weather. But unfortunately it apparently decreases game performance.
Regarding Fish: there are surely other things that can lurk underwater. Giant leeches or underwater plants with tentacles. Ok, we shouldn't discuss the topic 'tentacles' further...
There are also more things that could attack from above: Acid Rain, Gargoyle Zombies, Swarms of Black Flies that can make you blind for a few seconds when attacking. What about flying (Zombie) witches with broomsticks like in Painkiller?
Just some ideas...
By the way: Acid Zombies could be dressed as clowns. When they die, they pop balloons filled with poison gas.