What building themes do you think could still be done?

[Thylaco]

I haven't played the game yet, but Art Deco sort of strikes me as distinct from the existing, with plenty of statue variety too (might even be fun for a King Kong themed Gorilla enclosure).

Hausa architecture seems interesting, though maybe too similar to the North African scenery.

Cape Dutch seems like it might be approximated well by Classic. I've also seen a Farm theme being mentioned a fair bit.

 

[NachoB]

The África pack has a North African / Arabic theme. Not exactly like that but you can build similar things.

North American indeigenous art pieces, Mexican / Spanish colonial theme and Caribbean inspired building set are all good options.

 

[FlameWizard492]

Perhaps a Himalayan theme? Like something similar to monasteries and the villages that surround them?

 

[Aramar]

The one I've been asking for years...

 

[Thylaco]

Perhaps a Himalayan theme? Like something similar to monasteries and the villages that surround them?

I happened to make my own Nepalese scenery for ZT1 last year, Prayer flags are a pretty common element, though they are supposed to blow in the wind, I'm not sure if that's a thing in game.
I based my buildings on the Durbar (Royal) square in Kathmandu, some flags from Boudhanath Stupa & the boats from Phewa Lake. There's still a fair bit more to take influence from.
That said, I would think most of the buildings probably are well approximated between the Indian and East Asian sets.

Bhutan is also a potential influence considering the broader Himalayas.