On a technical level? I mean, yeah, it's possible in the strictly technical sense that they're able to do so. It'd probably be on the level of 'here let's add this to an upcoming patch' in terms of difficulty.
Are they likely to? I doubt. TBH, there's quite a lot of work that goes into making an animal- Modeling, texturing/rendering, building the AI, animating (especially if there's unique behaviors), all the research that has to go into making sure the in-game text and the animal's habitat requirements are accurate... And yeah, to an extent they can modify and re-skin existing animal frameworks to make new critters, but that's still a fair bit of work. Yes, there's a ton of work involved in making new foliage, build items, mechanics, and everything, but it's the animals that are the key selling point for a zoo game.
I think it's much more likely that if they revisit regions they've already done DLC for, they'll just make a new DLC pack. So what we'd functionally get is 'Arctic II', 'South America II', etc. Frankly, I'm cool with that. It's a great game, the DLC packs are really reasonably priced for the amount of additional content and gameplay mileage we get out of them, and I want Frontier to, y'know, be able to pay its dev teams and treat them well. I'm on board with giving them money for new animals.