Oh! Honestly, I actually headcanon that the Fire Nation is a combo of Japanese/Thai/Vietnamese, so I’m really glad that you (caught and) liked the inclusion!
I admit that I didn’t do a ton of research into the áo dài–I actually originally was going to put her in a Thai sinh (this one caught my eye), but changed my mind last minute after looking into sinh a little bit and then after seeing a couple pictures of áo dài (I really like these in particular).
Your ask prompted me to actually look into the history of the áo dài more (also because it’s not like…. 1:30 in the morning). Because I already had a Thai dress in my head and had glanced over a bit about them, where they tucked and tied and such to shape their clothes, that bled over into my internal design of Katara’s áo dài. The modern (from like… 1880s on it seems?) versions are the ones with zippers and knots, from what I can tell, as the basic design from Nguyen Dynasty on is about the same.
I found an interesting little article that briefly talks about the history of the áo dài, and I will say that I definitely imagine Katara’s dress being much like Fig. 4:
but in blues and such and probably less voluminous sleeves. Honestly all the pre-19th and 20th century designs are absolutely stunning in my opinion.
These definitely wouldn’t have zippers, but they also might not have any ties or knots (the article didn’t go in depth quite that much). I figure that with the Fire Nation (at least in my head) being an amalgamation of different culture of ours, that the styles would be an amalgamation as well. Thus combining the notion of tying etc from the brief look into traditional Thai clothing I did and the graceful lines of the áo dài to form layers of what I think some Fire Nation styles would be–squares of cloth layered and held together with maybe frog fasteners and internal ties (like kimono have) that make intricate layers as fitting nobility.
I hope that answers your curiosity somewhat?? Thank you for the ask!