I'm for this idea, like it was in AoI. However...
They would need a way to boost the animal spawn by a ton though for it to work. Seed lairs around the map that crank out various animals or something. As it is now even with the wildlands option turned on, all the animals are usually dead on the map by the time you get subdue animal.
Out of 10 games I think once I got a dancing bear, let alone a spider pen or anything else....
I don't like the idea of removing silk; I'd just as soon add a new "spider silk" resource. Silk is a useful resource from a game point of view, it's nice to add a couple commerce to a forrest tile.
I think that these should be a few animal dens placed on the map from the beginning, to make sure they don't all go extinct early on.
I was thinking that if we really want to keep the animal around longer, we shouldn't boost their spawn rate so much as place some of their dens on the impassible peaks (Bears and LotR type Giant Spiders seem like they would like to live in caves in the mountains anyway). Since Ranger, Beastmasters, and Druids can move through impassible terrain anyway, you could still destroy their dens in the mid to late game ,and these units would be more useful.
I agree, however placing the dens for spiders and bears in the mountains would ensure every city had a dancing bear and a spider pen (and therefore access to silk). Maybe the lairs in the mountains should be for a stronger animal with a lower spawn rate. You can still turn it into a "cage" but the animal would have a lesser impact on the later game. It may also give hunters a bit more of a challenge. You could even put a Hill Giant lair in the mountains...
As far as spiders and silk, on larger maps you don't always have access to silk so they are definately useful. I do like the idea of the pen granting the new resource "spider silk" as it would be a bonus to everyone who captures one (even if you are sitting next to 5 silks).