>Have Far Cry style island
>There are a set number of each animal on that island with a set reproduction rate
>Kill too much of X animal without defenses and only the poisonous or deadly ones survive making the game harder
>Kill too many of a food source animal and now you starve
>Each species has random traits like being big or small
>Smaller version of lets say rats are able to more easily get into the cracks of your gates and eat your food
>Bigger ones are harder to kill and do more damage to you
>Based on your play style the animals around you adapt
>Kill all the predators and suddenly the grazing animal you fed on has more dramatic population booms and busts making certain times more difficult
The closest I could think of was Wakfu where it had a natural ecosystem and killing too much of a certain animal or harvesting too much of a plant without planting more would make it go extinct on that server. They took it out once they realized players were fucking retarded and couldn't deal with that.