That being said, my brain hates me and decided to tell me to make a poorly drawn representation of what I believe it might look like at 2AM and then share it so you guys and gals could laugh at it.
http://i186.photobucket.com/albums/x247/derekkoch/Sketch.png
First, it donned on me that Argonians heads are very diverse, having fins or horns and maybe even feathers. It would most likely be highly expensive to arm any type of military with unique helms for every soldier. Barring the option to go helmless, I believe a generalized design that could be adapted quickly would be in order. Thus, the open-backed and layered plated design.
I feel I may have been a bit innacurate as to where the nostrils and eyes are. But it is poorly drawn, so the point is that a breathing slit and eye-holes would be necessary.
While having a solid face-plate, much of it is easily re-shaped plates layered upon eachother. Besides the easy fitting, these flat layers would also allow the armor to dynamically work with the Argonian's natural swimming ability.
Well, that's all I've got. The whole thing is up to Bethesda, of course, I just like wondering about the whats and ifs of stuff. I was also going to write a bit about what Khajit armor would be like, but I found myself imagining a variant of Redguard equipment.