Have the armor pieces (helmet, chestplate, leggings, boots) rendered in 3D. You could use the models used for the character, kinda how blocks just use their models to display what an item is.

I considered doing armor at one point, but the player model wearing the armor is always a 2d texture. Additionally, I'd rather not do armor because I would feel obligated to do so for modded armor sets as well.

I would suggest you try using this resource pack if you want to use 3d armor.