I was running Crimson Hammer and this idea randomly popped up in my brain. It's pretty simple.
"Darkfang Armband"
It is a piece of cloth (same color as the shade of yellow used by the Darkfang logo) worn around the armor's left arm. Maybe the color of the cloth would be in styles such as Cool, Divine, Volcanic, etc. to provide more customization.
The armband wouldn't be able to fit armors with "fat" sleeves, so 3-5* Skelly armors, Demo armors and Jelly armors are out unless they make a models specifically for said armors.
I don't know what Accessory slot it'll use, though.
Accessory - Darkfang Armband
Arm accessories would be pretty cool. You'd be able to wear things like SPIKES on your arms.
Of course, for this to happen, there'd need to be a new accessory slot implemented for arms. Certain armors won't be able to use it, sure, but I'd still be cool with it.
Plus one to this and arm accessories!
*Stares at Azure Guardian Armor, Almirian Crusader Armor and Snarbolax Coat.*
Azure Guardian Armor = Mighty Cobalt Armor + Cloth cape thing + Left shoulder armor + more knee guards
Snarbolax Coat = Wolver Coat + Left Shoulder Spike
Almirian Crusader Armor = Something with a cape and a fat shoulder plate
Theoretically your suggestion can be done. It should be a fairly large and obvious so it's not hidden by the shield (I never noticed Azure Guardian's left arm stuff until a few weeks ago). Might need it's own slot.
+1
Might be able to work - my only concern is that it'd need to be resized for fatter armours, as you mentioned, but I imagine a small armband would be pretty easily to model or just slap onto the texture of the armour.
That being said, I don't know if it'd be noticeable at all on the PC because of how small their arms are, not to mention it might be blocked by a large shield or weapon. Though even if this wasn't an issue, I don't think something that simple would be worth taking up an accessory slot, personally. Existing accessories have more bulk and size to them, and are also (technically) removed from the armour itself, being just a separate model that's well placed.
Though having SOME sort of arm accessory might be neat. Like a bandage or... something, I dunno.