It would be great if we could make hand held / wearable flags and banners with our coat of arms on them. That way, guilds, or individuals, could arrive at events or battles representing! These could also be purposeful for player venders.. Attracting attention.
I'd be happy with custom capes. The tech is already there, it could be easily implemented. Just upload your 150X150 image for approval. The image would go on the back of the cape of course and you could pick a custom color. Perhaps one crafter or one guild would have the rights to make the item. If there could be more, tabards would be great. It work in real after all.
Point is it would be easy even if you have to create a 1024X1024 image and submit a note from your great aunt and a trained squirrel. It is not like we have to design, code, rig and render a new star-ship (like Starcitizen testers are doing). *OK, sorry, I know it is actually hard to get a trained squirrel to help your great aunt write a note, but you get my drift.
I'm pretty sure this is why they are working with the College of Arms, so players will have their own custom heraldry that can be displayed in-game. Definitely would love to see things like lance banners, horse barding, personal baldrics, etc. May not make initial release, but I love custom personal stuff.