To be fair, would it really? Like, in terms of gameplay, what's the real difference between a looped bat and a looped parrot? How much would exhibit aquariums and maybe full on water based habitats really deviate from what we have to day?
Solely looking at gameplay, I think the game pretty much covers what there realistically can be covered in terms of building habitats.
I really disagree with that last sentence. I think there are new stuff which could be introduced ; mainly because both exhibit system are very, very rigid and they could be made way more flexible.
For the classical exhibit, instead of having pre-made biomes with little customization, you could just be given all of the exhibit logs and branches and every single piece, really, to build those biomes yourself. You could choose the size and the shape of your exhibit (to be able to do, for instance, round exhibits). I don't see any reason why this wouldn't be realistically possible in a zoo game.
I think that's quite important for small aquarias - for instance if I want to make a coral aquarium in my zoo I want to place the small coral pieces myself.
For the walthrough exhibit, I'd say the same kind of thing but with another scale. I want to be able to make it smaller and not-walkthrough because this would be the most common kind of aviary (still with looped behavior, that's fine by me) ; and I'd like to be able to make a different pathing as well (eg if you want an aviary people can enter in but can't fully cross).
And in particular, for birds, I'd like to be able to build mega-aviaries ; here in France, they are really common in many zoos (Beauval, Doué, Sigean, Villars-les-Dombes). So if you look at pictures of Doué's South American aviary, for instance, what you see is not single birds but a flock of birds. I'm pretty sure you could have this kind of mechanics in a zoo game, even with looped behaviour like those walkthrough exhibits, but on the scale of a flock of birds instead of a single bird.
I think this last point can realistically be covered in terms of building habitat ; yet isn't really covered at all.
To be clear, I don't think those are realistic expectations for Planet Zoo's DLCs, but I think there could very clearly be mechanical innovations in terms of exhibit (both classical and "walkthrough") building in a Planet Zoo 2. Especially in terms of modularity.