Would love to see faulds, tassets and/or ciusse added to protect the hip/waist/upper leg areas, as I've noticed that the majority of the red damage I take at Expert is from upper leg hits. Also, brigandines and coats of plates would be pretty awesome.
Some sort of trader needs to be added to the arena to make finding specific gear or specifically colored gear easier. I absolutely love the variety, just hate how hard it is to find matching stuff! (Speaking on this, I have literally never seen a purple padded coat/purple arming cap in 40+ hours of gameplay)
Also, something I'm curious about is the ranged combat in Exanima/SG. I don't think there's been any footage yet of it, which is kinda unfortunate as it's one of my most sought after features
I was thinking about how much it would affect the current arena meta last night. 2H users would have to rely on their armor as their only defense.
Edit 2-18-16:
Some other suggestions (I may add more with time):
The possibility for a helmet to be knocked off given a powerful enough blow and the right angle for such a thing to occur, as well as the option of strapping a helm to a cuirass.
A line of sight system that only allows your character to see roughly 175 degrees in front of him would be an interesting gameplay element; this ties into using helmets, as well, as I would suggest having those affect your visual cone. Areas that you have previously 'seen' but are not looking at that moment would still be visible, but darkened, and any movement in that area would not update or be seen until you actually looked in that direction again. I feel like this would add immersion to the game as well as give a needed negative to wearing a closed-face helm. Not sure if this has been previously suggested or not, if it has I apologize.
Jack chains in place of spaulders for use at aspirant and higher ranks. Really, really hoping this gets implemented as it's quite unique and they would serve a very good purpose in aspirant arena matches as well as early to early-1/3rd game armor.
Leather vambraces or bracers with metal bands for reinforcement.
Padded coats/gambesons with maille attached around the armpits and inner elbows, adding protection to areas that are exposed with plate armor.
And finally, my most poignant suggestion of them all; mod support. This game has potential oozing out of every orifice, and I can't even begin to imagine all the kinds of weapons/armor/arenas/maps/etc the community would churn out given the tools needed to get them in-game. I read somewhere that one thing putting the devs off from mod support (early on, at least) was exposing us to assets that aren't implemented yet. A very straightforward solution to this (in my opinion) is simply to remove the content you don't want us seeing, and when you want us to see it, include it in update data. Allowing the community to create their own content for others to use extends both the game's lifetime and the players interest in it, as well as providing the devs with possible game assets given that it is both a sufficient quality and that the creator(s) is okay with that(of which I'm sure they would be honored).
TL;DR: Mod support please.