The upcoming 1.14 update is a perfect chance to improve upon the design of every armor in the game (its tough being a blue rubber duck). The current textures are so bland and its saddening that horse armor is more visually appeasing. Note that there is already a "new" texture for armor, however the new design uses the foundation of the old texture. All of the armors listed here except leather need a more defined transition between armor plates instead of being strictly solid. The following list provides possible changes in armor texture.
Leather: Considering this set shares more of a likelihood to clothing instead of armor, a reduction to its physical bulkiness would be advisable. I don't think anything else really needs to be changed about leather armor as it may negatively affect minigames that use colored armor to identify teams.
Chainmail: Just like leather armor, chainmail needs to be less bulky because it is not a "heavy" armor like those listed below. Light plating on the chest and knees would be a nice addition, though.
Gold*: A royal theme with a red crown for the helmet and a jewel studded chestplate would be a preferable alternative to being a rubber duck.
Iron*: This set would need a darker gray in its color scheme instead of being a monotone white. A general medieval theme with a raised visor would fit iron armor the best.
Diamond*: Since this set is the most powerful in the game, it would be appropriate to give it a more "epic" theme, perhaps with geometric-appearing plates.
*Gold, iron, and diamond armor have an extended texture on their leggings so as to show a chainmail texture on the torso of the wearer for that armor type.
I'm not very skilled at pixel art so I don't know if I can provide examples.