I think the thing for me is that ... I love the animals and want to see many more of them (especially cooler climate North American, European or similar animals) but ... there's not much creation involved in them once you've built a habitat for them. Because of the remarkable complexity in the behaviors, they're fun to watch but there's not much interaction or activities to do with existing animals besides breeding them - and so far as I can tell, there's no real way to breed for specific colors aside from albino or melanistic black for those few animals (like the leopard) who has that (please, correct me if I'm wrong). Now, I'm here for the animals, and wouldn't for example buy something like planet coaster to build stuff for an amusement park - that just doesn't interest me. Most of the time lately, I play in sandbox and never even open the zoo - I really couldn't care less about the guests.
OTOH - the building pieces, like blocks, are endlessly variable so I can build a dozen different zoos with a completely different feel or theme. I don't tend to build 'zoos' exactly anyway, I often build a imaginary story, like my Lost City - which is a desert type place which ranges from a small desert town with a market place and a public bath, before travelling further to see the abandoned farm, the haunted graveyard, the serpent temple, etc. All of these are habitats with various animals, but I'm not trying to make them look like a particularly realistic zoo, but to fit with my sort of distant lands kind of theme. I have an ecotopia; a futuristic place ala 1970s idea of what an ideal future would look like, my Last Ark, set in Antarctica where survivors of some unknown environmental catastrophe are trying to save what animal species they can ... etc. And these all are uniqe built environments. Unlike most of the commenters, I like the themed pieces and would be happy to see more. I'd love to see more themes that focus on indigenous architecture (Mongolian yurts anyone?), or 1970s style brutalism, or art deco and so on.
In an ideal world, where we didn't have to anticipate Frontier sunsetting the game, I'd like to see the continuation of mixed DLCs with both animals and building pieces - or perhaps to split the two; animal focused DLCs, and scenery/building focused DLCs. Frontier could potentially get more money from those who buy both (as I would) and let folks chose what they like the most in their game.
Though I do agree, more generic zoo-like pieces are really needed for the game. Mesh screens, guest path airlock doors (for temperature controlled habitats), generic-ish animals signs (perhaps just expand the color scheme that was used for the caiman set to all the animals or something). I'd like some better way to build domes besides the current million piece method. And I still can't figure out why they included those hex glass pieces when there's no way to build geodesic domes with them.