While reading an article about the massive Phyton problem Florida faces and seeing a burst Phyton that tried to digest an alligator (pic of that at the bottom) I had some ideas:
what, if animals could eat each other? I mean right now they are all barbarians, but what if they had functions like "if animal=deer next to animal=sabertooth chancexx% to die; xx% chance to get damaged 30-60% etc" (could make a dynamic like: a lot of small fish enable growth of more sharks, lot of sharks eat themselves which in return again makes place for more small fish etc pp.)
That way 1. animal spawn rate could be higher as the animals reduce themselves - and certain "districts" would be established...
plus you wouldnt need a new fake civ for every animal so they can interact "naturally".
Another thing I thought about was the invasion of animals in new habitats like the asian phyton does in florida,,, could it be that spawning of animals shifts/variates a bit over time or via events "our hunter discovered a herd of XY migrating near our borders"/"contact with a new civXY brought a plague of their beloved holy rats on our fields"?
I found out that though I like the authenticy to have kangeroos only in southern hemisphere killing the wolverine No 1000 isn't so much fun at all ;-)
Last but not least: a request: I liked some menace animals like rats, frogs and grasshoppers, even mosquito. frog and rat could even be subdueable (maybe even a flea for a flea circus building lol) and being able to enter cultural borders in masses to block and pillage improvements.
It could take a while to take out 10 or so grasshoppers/rats/frogs on a tile... - they could start to invade after agriculture (and monotheism^^) was discoverd maybe...