I don't really see a need for any additional "themes" tbh. The SEA pack didn't add any specific building pieces to Indonesia, Cambodia, Vietnam or Malaysia style architecture, and at first people complained, yet a few weeks after release we see it's very easy to build in those styles with pieces we already have. People can make do with what we already have.
If I had to pick one, a limited NA building set with some generic (and respectful) First Nations artwork, particularly totems from the Pacific NW tribes, would be nice. A western theme doesn't seem needed, especially because if we got a NA pack and it was only a 4 + 1, there's no way they're adding enough desert animals to justify adding an entire set.
What we do definitely need however (and I don't consider this a "theme" per se), is mesh and chain link walls and roof pieces, more rope pieces, more natural looking climbing frame pieces, and some behind the scenes style props. Things that would actually make a zoo feel like a zoo and not a theme park, things you could use in any zoo in any section, so basically, not incredibly niche like some of the other stuff we've seen (looking at you, SA bins, benches and tables).