Biome or not, I still think a coastal pack would go insanely hard. Not as our last pack, but maybe for a Planet Zoo 2 or something.
A scenery pack with say, a sea otter, a walrus, a pelican, and some sort of oddball shorebird like a pied avocet. Maybe some sort of crustacean - like the fan-favorite coconut crab - as an exhibit animal. Two unique, crowd pleasing carnivorans that aren't a cat, dog, or bear, two banger birds, and an exhibit species unlike any other.
The scenery could be nice too, even if it might be stuff we could already do in game with more pieces. Some nice, colorful clapboard for instance:
View attachment 389057View attachment 389058
Or of course, the foliage could really sell the set here. Some more beach grass species, some nice, weathered rocks, coastal pines and/or palms, more aquatic plants, etc:
View attachment 389059View attachment 389060
View attachment 389063View attachment 389062
Alternatively, as an animal pack, ditch the scenery and add:
A seaduck species! A harlequin duck, spectacled eider, or a Chinese merganser for example.
The American flamingo. Another bird? Yeah, but its a "clone" and most of the best picks here are birds anyways, sue me.
The green sea turtle. I'll be real, I'm not a fan of this one as it would be feel out of place without the fully aquatic species that often accompany it, but it was the only option I could think of that wasn't another bird (I'm already pushing what's realistic lol) and I know it would make a lot of people happy.
If we want to push the definition of coastal to animals that are just sometimes found there, but not really associated with it nor permanently live there often, I certainly wouldn't mind shoehorning in an echidna lol. A gray fox could be one that comes to mind too - not my preferred option at all but certainly feels like one Frontier might throw in, especially if there are too many new rigs already.
Other picks could include a puffin species, - which I only didn't include due to debates as to whether they would look strange without flying as habitat animals/whether birds will ever appear in the walkthrough - or turning the coconut crab into a habitat animal.

I mean this would probably be one of my favorite packs ever. It would have my vote.