To be fair, keep in mind that Zoo Tycoon 2013 was made under the suffocating limitations of Microsoft at the time. And as for the kind of DLC they're working on, we don't know how they're splitting the work and what they prioritize. The Arctic DLC was barely a month after release, so I'm not too surprised it was very limited. Consider, for instance, they might have different people working on A: completely new animals, and B: "recolours/reworks" of old animals (like the Arctic wolf was). Like, more experienced employees for the former, newbies for the latter. The brand new, unique animals take longer to make, but can be withheld until there's enough of them to make a truly interesting DLC pack.
I don't want to be too optimistic, but Frontier described PZ as "the game we always wanted to make" (while making ZT 2013). And if Planet Coaster is anything to go by, we have 2-3 years of DLC coming, spread over about 12 packs. Plus, studios tend to get better at making games over time.
Me, I kinda hope there's more than 4 animals for some of the packs, because there's SO many great animals to put in the game. I'd also sacrifice new building pieces for a big aquatic animals expansion DLC, without question.