I've been a proponent of this idea for a long time. It could open things up in a very meaningful way, as far as what species are able to fit into packs or not. And it could be a nice way to add animals to the game that might be tricky to fit into other packs, have a niche audience, etc. Beyond that? It would add another "hype cycle" when we're about halfway toward a bigger DLC -- and further encourage content creators.
The examples I always point to are Madagascar and mustelids. There are a handful of animals from Madagascar that people would really want in-game, but there aren't quite enough to justify a full animal DLC. But a smaller pack with 1-2 new lemur species, the fossa, and a mongoose or civet? I bet that would sell really well! Sure, some of these could be incorporated into an "islands" or "rainforest" pack but... This would allow that region to be fully fleshed out - and allow more space for animals in that bigger pack. It's a win-win.
With mustelids, imagine a pack with the honey badger, wolverine, NA river otter (or sea otter), black-footed ferret or a stoat of some sort, and the honey badger. Most of these are really wanted by the community! And again, they could fit into another potential DLC, but the options are limited given that these would have all had slots in existing DLCs.
All said, I think packs like this could work really well to "fill gaps" and put animals into the mix that might not see the light of day because their "chance" has already happened.
I'll even take it a step further and suggest that they could do something similar for scenery pieces! No animals, just scenery. 75-100 items for $5-10 or whatever. And there's loads of potential here too! A vintage zoo/menagerie building set and props. An 80s-90s style zoo building set. New sign sets for base game and early DLC animals that match the current art style. Prop packs centered around various themes: Parks and gardens, barnyards, cities and towns (building facades, parking lots, street lines that could snap to the center of paths), gift shops and restaurant decor. The list goes on!
These are all things that could add even more life to the game than Frontier already has, would be easier to put together as they wouldn't be hyper-themed, and could just... Really expand the game.
So yeah, this is a great idea and I'm here for it!